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HNSC 3162 Biological Concepts in Public Health (Cai)

Professor Patricia Cai OER

Topic 1: Introduction to Public Health

Learning Objectives:

learning outcomes icon.

By the end of this section, you will be able to:

  • define health.
  • describe the purpose of public health.
  • define key terms used in public health.
  • identify prominent events in the history of public health.
  • recognize the core public health functions and services.
  • describe the role of different stakeholders in the field of public health.
  • list some determinants of health.
  • recognize how individual determinants of health affect population health.


Citation: Let's Learn Public Health. (Jun 15, 2017) What is Public Health??. (5:33) URL:

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Topic 2: Selected Concepts of Public Health

Learning Objectives:

learning outcomes icon.

By the end of this section, you will be able to:

  • explain what epidemiology studies and define etiology.
  • explain the difference between prevalence and incidence of disease. 
  • distinguish the characteristics of sporadic, endemic, epidemic, and pandemic diseases.
  • explain the use of Koch’s postulates and their modifications to determine the etiology of disease.
  • explain the mission of CDC.


The field of epidemiology concerns the geographical distribution and timing of infectious disease occurrences and how they are transmitted and maintained in nature, with the goal of recognizing and controlling outbreaks. The science of epidemiology includes etiology (the study of the causes of disease) and investigation of disease transmission (mechanisms by which a disease is spread).

Analyzing Disease in a Population

Epidemiological analyses are always carried out with reference to a population, which is the group of individuals that are at risk for the disease or condition. The population can be defined geographically, but if only a portion of the individuals in that area are susceptible, additional criteria may be required. Susceptible individuals may be defined by particular behaviors, such as intravenous drug use, owning particular pets, or membership in an institution, such as a college. Being able to define the population is important because most measures of interest in epidemiology are made with reference to the size of the population.

The state of being diseased is called morbidity. Morbidity in a population can be expressed in a few different ways. Morbidity or total morbidity is expressed in numbers of individuals without reference to the size of the population. The morbidity rate can be expressed as the number of diseased individuals out of a standard number of individuals in the population, such as 100,000, or as a percent of the population.

There are two aspects of morbidity that are relevant to an epidemiologist: a disease’s prevalence and its incidence. Prevalence is the number, or proportion, of individuals with a particular illness in a given population at a point in time. For example, the Centers for Disease Control and Prevention (CDC) estimated that in 2012, there were about 1.2 million people 13 years and older with an active human immunodeficiency virus (HIV) infection. Expressed as a proportion, or rate, this is a prevalence of 467 infected persons per 100,000 in the population.1 On the other hand, incidence is the number or proportion of new cases in a period of time. For the same year and population, the CDC estimates that there were 43,165 newly diagnosed cases of HIV infection, which is an incidence of 13.7 new cases per 100,000 in the population.2 The relationship between incidence and prevalence can be seen in Figure 16.2external link.. For a chronic disease like HIV infection, prevalence will generally be higher than incidence because it represents the cumulative number of new cases over many years minus the number of cases that are no longer active (e.g., because the patient died or was cured).

In addition to morbidity rates, the incidence and prevalence of mortality (death) may also be reported. A mortality rate can be expressed as the percentage of the population that has died from a disease or as the number of deaths per 100,000 persons (or other suitable standard number).

figure 16.2 Microbiology book's Figure 16.2 This graph compares the incidence of HIV (the number of new cases reported each year) with the prevalence (the total number of cases each year). Prevalence and incidence can also be expressed as a rate or proportion for a given population.


Citation: Patwari, Rahul.(Jul 26, 2013) The Relationship Between Incidence and Prevalence (3:07) URL:

Patterns of Incidence

Diseases that are seen only occasionally, and usually without geographic concentration, are called sporadic diseases. Examples of sporadic diseases include tetanus, rabies, and plague. In the United States, Clostridium tetani, the bacterium that causes tetanus, is ubiquitous in the soil environment, but incidences of infection occur only rarely and in scattered locations because most individuals are vaccinated, clean wounds appropriately, or are only rarely in a situation that would cause infection.3 Likewise in the United States there are a few scattered cases of plague each year, usually contracted from rodents in rural areas in the western states.4

Diseases that are constantly present (often at a low level) in a population within a particular geographic region are called endemic diseases. For example, malaria is endemic to some regions of Brazil, but is not endemic to the United States.

Diseases for which a larger than expected number of cases occurs in a short time within a geographic region are called epidemic diseases. Influenza is a good example of a commonly epidemic disease. Incidence patterns of influenza tend to rise each winter in the northern hemisphere. These seasonal increases are expected, so it would not be accurate to say that influenza is epidemic every winter; however, some winters have an usually large number of seasonal influenza cases in particular regions, and such situations would qualify as epidemics (Figure 16.3 external link.and Figure 16.4 external link.).

An epidemic disease signals the breakdown of an equilibrium in disease frequency, often resulting from some change in environmental conditions or in the population. In the case of influenza, the disruption can be due to antigenic shift or drift (see Virulence Factors of Bacterial and Viral Pathogens external link.), which allows influenza virus strains to circumvent the acquired immunity of their human hosts.

An epidemic that occurs on a worldwide scale is called a pandemic disease. For example, HIV/AIDS is a pandemic disease and novel influenza virus strains often become pandemic.

figure 16.3

Microbiology book's Figure 16.3 The 2007–2008 influenza season in the United States saw much higher than normal numbers of visits to emergency departments for influenza-like symptoms as compared to the previous and the following years. (credit: modification of work by Centers for Disease Control and Prevention)


Figure 16 4 The seasonal epidemic threshold (blue curve.)

Microbiology book's Figure 16.4 The seasonal epidemic threshold (blue curve) is set by the CDC-based data from the previous five years. When actual mortality rates exceed this threshold, a disease is considered to be epidemic. As this graph shows, pneumonia- and influenza-related mortality saw pronounced epidemics during the winters of 2003–2004, 2005, and 2008. (credit: modification of work by Centers for Disease Control and Prevention)



When studying an epidemic, an epidemiologist’s first task is to determinate the cause of the disease, called the etiologic agent or causative agent. Connecting a disease to a specific pathogen can be challenging because of the extra effort typically required to demonstrate direct causation as opposed to a simple association. It is not enough to observe an association between a disease and a suspected pathogen; controlled experiments are needed to eliminate other possible causes. In addition, pathogens are typically difficult to detect when there is no immediate clue as to what is causing the outbreak. Signs and symptoms of disease are also commonly nonspecific, meaning that many different agents can give rise to the same set of signs and symptoms. This complicates diagnosis even when a causative agent is familiar to scientists.

Robert Koch was the first scientist to specifically demonstrate the causative agent of a disease (anthrax) in the late 1800s. Koch developed four criteria, now known as Koch’s postulates, which had to be met in order to positively link a disease with a pathogenic microbe. Without Koch’s postulates, the Golden Age of Microbiology would not have occurred. Between 1876 and 1905, many common diseases were linked with their etiologic agents, including cholera, diphtheria, gonorrhea, meningitis, plague, syphilis, tetanus, and tuberculosis. Today, we use the molecular Koch’s postulates, a variation of Koch’s original postulates that can be used to establish a link between the disease state and virulence traits unique to a pathogenic strain of a microbe. Koch’s original postulates and molecular Koch’s postulates were described in more detail in How Pathogens Cause Disease.external link.

The Role of Public Health Organizations

The main national public health agency in the United States is the Centers for Disease Control and Prevention (CDC), an agency of the Department of Health and Human Services. The CDC is charged with protecting the public from disease and injury. One way that the CDC carries out this mission is by overseeing the National Notifiable Disease Surveillance System (NNDSS) in cooperation with regional, state, and territorial public health departments. The NNDSS monitors diseases considered to be of public health importance on a national scale. Such diseases are called notifiable diseases or reportable diseases because all cases must be reported to the CDC. A physician treating a patient with a notifiable disease is legally required to submit a report on the case. Notifiable diseases include HIV infection, measles, West Nile virus infections, and many others. Some states have their own lists of notifiable diseases that include diseases beyond those on the CDC’s list.

Notifiable diseases are tracked by epidemiological studies and the data is used to inform health-care providers and the public about possible risks. The CDC publishes the Morbidity and Mortality Weekly Report (MMWR), which provides physicians and health-care workers with updates on public health issues and the latest data pertaining to notifiable diseases.

Chain of Infection (CDC and Ottawa Public Health)

The traditional epidemiologic triad model holds that infectious diseases result from the interaction of agent, host, and environment. More specifically, transmission occurs when the agent leaves its reservoir or host through a portal of exit, is conveyed by some mode of transmission, and enters through an appropriate portal of entry to infect a susceptible host. This sequence is sometimes called the chain of infection. (See figure below)

chain of infection.

Chain of Infection Figure Description:

Germs (agent)
  • Bacteria
  • Viruses
  • Parasites
Where germs live (reservoir)
  • People
  • Animals/Pets (dogs, cats, reptiles)
  • Wild animals
  • Food
  • Soil
  • Water
How germs get out (portal of exit)
  • Mouth (vomit, saliva)
  • Cuts in the skin (blood)
  • During diapering and toileting stool
Germs get around (mode of transmission)
  • Contact (hands, toys, sand)
  • Droplets (when you speak, sneeze or cough)
How germs get in (portal of entry)
  • Mouth
  • Cuts in the skin
  • Eyes
Next sick person (susceptible host)
  • Babies
  • Children
  • Elderly
  • People with a weakened immune system
  • Unimmunized people
  • Anyone

The Cycle Repeats

Go to Next Selected Concepts of Public Health:

Learning Objectives:

learning outcomes icon.By the end of this section, you will be able to:

  • define health promotion.
  • describe the significance of Ottawa charter and identify examples in each key component.


More than 200 participants from 38 countries met in November 1986 in Ottawa to exchange experiences and share knowledge of health promotion. The conference stimulated an open dialogue among health workers. politicians, academics and representatives of governmental. voluntary and community organizations. The charter they drew up reflected their individual and collective commitment to the common goal of Health for all by the year 2000.

Citation: Let's Learn Public Health (Mar 4, 2017) Health Promotion and the Ottawa Charter - Creating Healthier Populations. (5:46) URL:

Knowledge Check:

Go to Next Selected Concepts of Public Health:

Learning Objectives:

learning outcomes icon.By the end of this section, you will be able to:

  • describe what social determinants of health are.
  • identify elements in the structural and intermediary social determinants as developed by WHO.


Citation: Let's Learn Public Health (Jun 25, 2017) What Makes Us Healthy? Understanding the Social Determinants of Health. (6:27). URL:

Executive Summary

Complexity defines health. Now, more than ever, in the age of globalization, is this so. The Commission on Social Determinants of Health (CSDH) was set up by the World Health Organization (WHO) to get to the heart of this complexity. They were tasked with summarizing the evidence on how the structure of societies, through myriad social interactions, norms and institutions, are affecting population health, and what governments and public health can do about it. To guide the Commission in its mammoth task, the WHO Secretariat conducted a review and summary of different frameworks for understanding the social determinants of health. This review was summarized and synthesized into a single conceptual framework for action on the social determinants of health which was proposed to and, largely, accepted by, the CSDH for orienting their work. A key aim of the framework is to highlight the difference between levels of causation, distinguishing between the mechanisms by which social hierarchies are created, and the conditions of daily life which then result. This paper describes the review, how the proposed conceptual framework was developed, and identifies elements of policy directions for action implied by the proposed conceptual framework and analysis of policy approaches.

A key lesson from history (including results from the previous “historical” paper - see Discussion Paper 1 in this Series), is that international health agendas have tended to oscillate between: a focus on technology-based medical care and public health interventions, and an understanding of health as a social phenomenon, requiring more complex forms of intersectoral policy action. In this context, the Commission’s purpose was to revive the latter understanding and therein WHO’s constitutional commitments to health equity and social justice.

Having health framed as a social phenomenon emphasizes health as a topic of social justice more broadly. Consequently, health equity (described by the absence of unfair and avoidable or remediable differences in health among social groups) becomes a guiding criterion or principle. Moreover, the framing of social justice and health equity, points towards the adoption of related human rights frameworks as vehicles for enabling the realization of health equity, wherein the state is the primary responsible duty bearer. In spite of human rights having been interpreted in individualistic terms in some intellectual and legal traditions, notably the Anglo-Saxon, the frameworks and instruments associated with human rights guarantees are also able to form the basis for ensuring the collective well-being of social groups. Having been associated with historical struggles for solidarity and the empowerment of the deprived they form a powerful operational framework for articulating the principle of health equity.

Theories on the social production of health and disease

With this general framing in mind, developing a conceptual framework on social determinants of health (SDH) for the CSDH needs to take note of the specific theories of the social production of health. Three main theoretical non-mutually exclusive explanations were reviewed: (1) psychosocial approaches; (2) social production of disease/political economy of health; and (3) eco-social frameworks.

All three of these theoretical traditions, use the following main pathways and mechanisms to explain causation: (1) “social selection”, or social mobility; (2) “social causation”; and (3) life course perspectives. Each of these theories and associated pathways and mechanisms strongly emphasize the concept of “social position”, which is found to play a central role in the social determinants of health inequities.

A very persuasive account of how differences in social position account for health inequities is found in the Diderichsen’s model of “the mechanisms of health inequality”. Didierichsen’s work identifies how the following mechanisms stratify health outcomes:

  • Social contexts, which includes the structure of society or the social relations in society, create social stratification and assign individuals to different social positions.
  • Social stratification in turn engenders differential exposure to health-damaging conditions and differential vulnerability, in terms of health conditions and material resource availability.
  • Social stratification likewise determines differential consequences of ill health for more and less advantaged groups (including economic and social consequences, as well differential health outcomes per se).

The role of social position in generating health inequities necessitates a central role for a further two conceptual clarifications. First, the central role of power. While classical conceptualizations of power equate power with domination, these can also be complemented by alternative readings that emphasize more positive, creative aspects of power, based on collective action as embodied in legal system class suits. In this context, human rights embody a demand on the part of oppressed and marginalized communities for the expression of their collective social power. The central role of power in the understanding of social pathways and mechanisms means that tackling the social determinants of health inequities is a political process that engages both the agency of disadvantaged communities and the responsibility of the state. Second, it is important to clarify the conceptual and practical distinction between the social causes of health and the social factors determining the distribution of these causes between more and less advantaged groups. The CSDH framework makes a point of making clear this distinction.

On this second point of clarification, conflating the social determinants of health and the social processes that shape these determinants’ unequal distribution can seriously mislead policy. Over recent decades, social and economic policies that have been associated with positive aggregate trends in health determining social factors (e.g. income and educational attainment) have also been associated with persistent inequalities in the distribution of these factors across population groups. Furthermore, policy objectives are defined quite differently, depending on whether the aim is to address determinants of health or determinants of health inequities.

The CSDH Conceptual Framework

Bringing these various elements together, the CSDH framework, summarized in Figure A, shows how social, economic and political mechanisms give rise to a set of socioeconomic positions, whereby populations are stratified according to income, education, occupation, gender, race/ethnicity and other factors; these socioeconomic positions in turn shape specific determinants of health status (intermediary determinants) reflective of people’s place within social hierarchies; based on their respective social status, individuals experience differences in exposure and vulnerability to health-compromising conditions. Illness can “feed back” on a given individual’s social position, e.g. by compromising employment opportunities and reducing income; certain epidemic diseases can similarly “feed back” to affect the functioning of social, economic and political institutions.

“Context” is broadly defined to include all social and political mechanisms that generate, configure and maintain social hierarchies, including: the labour market; the educational system, political institutions and other cultural and societal values. Among the contextual factors that most powerfully affect health are the welfare state and its redistributive policies (or the absence of such policies). In the CSDH framework, structural mechanisms are those that generate stratification and social class divisions in the society and that define individual socioeconomic position within hierarchies of power, prestige and access to resources. Structural mechanisms are rooted in the key institutions and processes of the socioeconomic and political context.

The most important structural stratifiers and their proxy indicators include: Income, Education, Occupation, Social Class, Gender, Race/ethnicity.

Together, context, structural mechanisms and the resultant socioeconomic position of individuals are “structural determinants” and in effect it is these determinants we refer to as the “social determinants of health inequities.” The underlying social determinants of health inequities operate through a set of intermediary determinants of health to shape health outcomes. The vocabulary of “structural determinants” and “intermediary determinants” underscores the causal priority of the structural factors. The main categories of intermediary determinants of health are: material circumstances; psychosocial circumstances; behavioral and/or biological factors; and the health system itself as a social determinant.

  • Material circumstances include factors such as housing and neighborhood quality, consumption potential (e.g. the financial means to buy healthy food, warm clothing, etc.), and the physical work environment.
  • Psychosocial circumstances include psychosocial stressors, stressful living circumstances and relationships, and social support and coping styles (or the lack thereof).
  • Behavioral and biological factors include nutrition, physical activity, tobacco consumption and alcohol consumption, which are distributed differently among different social groups. Biological factors also include genetic factors.

The CSDH framework departs from many previous models by conceptualizing the health system itself as a social determinant of health (SDH). The role of the health system becomes particularly relevant through the issue of access, which incorporates differences in exposure and vulnerability, and through intersectoral action led from within the health sector. The health system plays an important role in mediating the differential consequences of illness in people’s lives.

CSDH conceptual framework

The concepts of social cohesion and social capital occupy a conspicuous (and contested) place in discussions of SDH. Social capital cuts across the structural and intermediary dimensions, with features that link it to both. Yet focus on social capital, depending on interpretation, risks reinforcing depoliticized approaches to public health and the SDH, when the political nature of the endeavour needs to be an explicit part of any strategy to tackle the SDH. Certain interpretations have not depoliticized social capital, notably the notion of “linking social capital”, which have spurred new thinking on the role of the state in promoting equity, wherein a key task for health politics is nurturing cooperative relationships between citizens and institutions. According to this literature, the state should take responsibility for developing flexible systems that facilitate access and participation on the part of the citizens.

Policy action

Finally, in turning to policy action on SDH inequities, three broad approaches to reducing health inequities can be identified. These may be based on: (1) targeted programmes for disadvantaged populations; (2) closing health gaps between worse-off and better-off groups; and (3) addressing the social health gradient across the whole population. A consistent equity-based approach to SDH must ultimately lead to a gradients focus. However, strategies based on tackling health disadvantage, health gaps and gradients are not mutually exclusive. They can complement and build on each other.

Policy development frameworks can help analysts and policymakers to identify levels of intervention and entry points for action on SDH, ranging from policies tackling underlying structural determinants to approaches focused on the health system and reducing inequities in the consequences of ill health suffered by different social groups. The review showed the framework that Diderichsen and colleagues proposed- a typology or mapping of entry points for policy action on SDH inequities - to be very useful in the way it is very closely aligned to theories of causation. They identify actions related to: social stratification; differential exposure/ differential vulnerability; differential consequences and macro social conditions.

Considerations of these policy action frameworks lead to discussion of three key strategic directions for policy work to tackle the SDH, with a particular emphasis on tackling health inequities: (1) the need for strategies to address context; (2) intersectoral action; and (3) social participation and empowerment.

Policy action challenges for the CSDH

Arguably the single most significant lesson of the CSDH conceptual framework is that interventions and policies to reduce health inequities must not limit themselves to intermediary determinants, but must include policies specifically crafted to tackle the social mechanisms that systematically produce an inequitable distribution of the determinants of health among population groups (see Figure B). To tackle structural, as well as intermediary, determinants requires intersectoral policy approaches. 8

Framework for tackling SDH inequities

A key task for the CSDH will be:

  1. to identify successful examples of intersectoral action on SDH in jurisdictions with different levels of resources and administrative capacity; and to characterize in detail the political and management mechanisms that have enabled effective intersectoral programmes to function sustainably.
  2. to demonstrate how participation of civil society and affected communities in the design and implementation of policies to address SDH is essential to success. Empowering social participation provides both ethical legitimacy and a sustainable base to take the SDH agenda forward after the Commission has completed its work.
  3. Finally, SDH policies must be crafted with careful attention to contextual specificities, which should be rigorously characterized using methodologies developed by social and political science.

Knowledge Check:

Go to Next Selected Concepts of Public Health:

Learning Objectives:

learning outcomes icon.By the end of this section, you will be able to:

  • define health literacy.
  • describe why health literacy matter


What is Health Literacy?

In the report, Healthy People external link., the U.S. Department of Health and Human Services included improved consumer health literacy and identified health literacy as an important component of health communication, medical product safety, and oral health. Health literacy is defined in Healthy People as: “The degree to which individuals have the capacity to obtain, process, and understand basic health information and services needed to make appropriate health decisions.”

Health literacy includes the ability to understand instructions on prescription drug bottles, appointment slips, medical education brochures, doctor’s directions and consent forms, and the ability to negotiate complex health care systems. Health literacy is not simply the ability to read. It requires a complex group of reading, listening, analytical, and decision-making skills, and the ability to apply these skills to health situations.

Health literacy varies by context and setting and is not necessarily related to years of education or general reading ability. A person who functions adequately at home or work may have marginal or inadequate literacy in a health care environment. With the move towards a more “consumer-centric” health care system as part of an overall effort to improve the quality of health care and to reduce health care costs, individuals need to take an even more active role in health care related decisions. To accomplish this people need strong health information skills.

Why Does Health Literacy Matter?

Every day, people confront situations that involve life-changing decisions about their health. These decisions are made in places such as grocery and drug stores, workplaces, playgrounds, doctors’ offices, clinics and hospitals, and around the kitchen table. Obtaining, communicating, processing, and understanding health information and services are essential steps in making appropriate health decisions; however, research indicates that today’s health information is presented in ways that are not usable by most adults. “Limited health literacy” occurs when people can’t find and use the health information and services they need.

  • Nearly 9 out of 10 adults have difficulty using the everyday health information that is routinely available in our healthcare facilities, retail outlets, media, and communities.
  • Without clear information and an understanding of the information’s importance, people are more likely to skip necessary medical tests, end up in the emergency room more often, and have a harder time managing chronic diseases like diabetes or high blood pressure.

Whom Does Health Literacy Affect?

People of all ages, races, incomes, and educational levels can find it difficult to obtain, communicate, process, and understand health information and services. Literacy skills are only a part of health literacy. Even people with strong reading and writing skills can face health literacy challenges when they are not familiar with medical terms or how their bodies work. They have to interpret or calculate numbers or risks that could have immediate effects on their health and safety. They are diagnosed with a serious illness and are scared or confused. They have health conditions that require complicated self-care. Every organization involved in health information and services needs its own health literacy plan to improve its organizational practices to help everyone of all ages.

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