While the clinician is primarily concerned with disease, the epidemiologist is interested in both infection and disease. The host responses can include death, the classic clinical features of the disease, mild or atypical forms, subclinical and inapparent infections, and the carrier state, which may exist in the absence of a detectable host response.

These factors that result in the occurrence of clinical illness among those infected have been called the “clinical illness-promotion factors,”(3) and many of them remain unknown. These intrinsic factors include the age at the time of infection, the portal of entry, the presence or absence of immunity, the vigor of the primary defense system, the efficiency and nature of humoral and cell-mediated immune responses, the genetic makeup of the host, the state of nutrition, the presence of other diseases, and psychosocial influences. Disease represents one of the possible consequences of infection, and the factors important in its development are mostly intrinsic to the host, although the dosage and virulence of the infecting microbe play a role. Exposure is the key factor, and the sources of infection lie mostly outside the individual human host, within the environment, or in other infected hosts. Infection is the consequence of an encounter of a potentially pathogenic microorganism with a susceptible human host through an appropriate portal of entry and usually involves a demonstrable host response to the agent. For infectious diseases, one must study the circumstances under which both infection and disease occur, for these may be different. Epidemiology is the study of the distribution and determinants of health-related states, conditions, or events in specified populations and the application of the results of this study to the control of health problems.(1) It is a quantitative science concerned in infectious diseases with the circumstances under which disease processes occur, the factors that affect their incidence and the host response to the infectious agent, and the use of this knowledge for control and prevention.(2) It includes the pathogenesis of disease in both the community and the individual.

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