The objectives of epidemiology include the ability to:

• identify the etiology or cause of disease
• determine the extent of disease
• study the progression of the disease
• evaluate preventive and therapeutic measures for a disease or condition
• develop public health policy

One objective of epidemiology is to identify the cause of a disease, with a desire to prevent or modify the severity of the condition. Consider the table below. Would you agree that this table accurately portrays the true causes of death in the U.S. population? Why or why not?

*Composite approximation drawn from studies that use different approaches to derive estimates, ranging from actual counts (eg, firearms) to population attributable risk calculations (eg, tobacco). Numbers over 100,000 rounded to the nearest 100 000; over 50 000, rounded to the nearest 10,000; below 50,000, rounded to the nearest 5000.

Table: Estimated numbers by 'Cause' of Death(From McGinnis JM, Foege, WH. 1993 JAMA, 270(18): 2207-2212.)

As you may have noticed, the causes of death in Table 1 are all related to modifiable factors. The percentages do not total 100, but if these results are accurate, a large percentage of deaths can be postponed. The opportunity to prevent or ameliorate disease is an exciting component of epidemiologic study.
Epidemiologists follow pre-determined procedures in deciding whether to attribute a particular factor as a cause of a disease or condition. In the late 19th century, a German microbiologist, Robert Koch, devised a scheme for deciding whether or not a particular microbe caused a disease.

Koch's Postulates

One organism leads to one disease. (one-to-one)

• A specific organism must always be observed in association with the disease. (regular presence)
• The organism must be isolated from an infected host and grown in pure culture in the laboratory. (exclusive presence)
• When organisms from the pure culture are inoculated into a susceptible host organism, it must cause the disease. (sufficient cause)
• The infectious organism must be re-isolated from the diseased organism and grown in pure culture.

  Do you see any problem with applying Koch's postulates to determine the cause of all diseases?

  Consider asthma or lung cancer: can one micro-organism be isolated as causing the development of these conditions?

Modern Epidemiology

Modern epidemiology accommodates multiple exposures contributing to increased risk for one disease (many-to-one) and situations where one risk factor contributes to multiple diseases (one-to-many).

Considerations When Assessing Possible Causal Role of a Risk

Obviously, there are many factors to assess when considering whether a potential risk factor causes a disease or condition:

• How strong is the association? (odds ratio, relative risk)
• Is there a dose-response relationship?
• If exposure ceases, what happens? Does the condition change?
• Can the findings be replicated?
• Is there biological plausibility?
• Are there alternative explanations?
• How specific is the association?
• Is this consistent with other knowledge?
• Is there a statistical association? If so, is the association
• Spurious, due to chance or bias
• Non-causal OR Causal?
• Is a temporal relationship observed?
• Was the study design adequate?

A traditional model of infectious disease causation, known as the Epidemiologic Triad is depicted in Figure 2. The triad consists of an external agent, a host, and an environment in which the host and agent are brought together, causing the disease to occur in the host. A vector, an organism that transmits infection by conveying the pathogen from one host to another without causing the disease itself, could be part of the infectious process.

A classic example of a vector is the Anopheles mosquito. As the mosquito ingests blood from an infected host, it picks up the parasite plasmodium. The plasmodium is harmless to the mosquito. However, after being stored in the salivary glands and then injected into the next human upon which the mosquito feeds, the plasmodium can cause malaria in the infected human. Thus, the Anopheles mosquito serves as a vector for malaria. Another familiar example of a vector is ticks of the genus Ixodes which can be vectors for Lyme disease.

In the traditional epidemiologic triad model, transmission occurs when the agent leaves its reservoir or host through a portal of exit and is conveyed by a mode of transmission to enter through an appropriate portal of entry to infect a susceptible host. Transmission may be direct (direct contact host-to-host, droplet spread from one host to another) or indirect (the transfer of an infectious agent from a reservoir to a susceptible host by suspended air particles, inanimate objects (vehicles or fomites), or animate intermediaries (vectors).

Can the epidemiologic triad be applied to a disease that is not infectious? Consider a smoking-related disease (Figure 3). If smoking (or more specifically, a carcinogen in the smoke of the cigarette) causes the disease, those who manufacture, sell and distribute cigarettes are vectors, bringing the disease-causing agent to the susceptible host. Diagramming the epidemiologic triad also indicates potential interventions to reduce disease in the population. In this example, clean indoor air legislation, advertising potential harm from smoking, or establishing workplace smoking cessation programs could change the environment and reduce the exposure of the host to the agent. Conversely, increased advertising from cigarette manufacturers or increased numbers of vendors would increase the exposure of the host to the agent.

Thus, the traditional model of disease transmission can be useful to identify areas of potential intervention to reduce disease prevalence, whether infectious or non-infectious.