Infection and Disease

INFECTION AND DISEASE

An important consideration in the study of the epidemiology of communicable organisms is the distinction between infection and disease. Infection involves multiplication of the organism in or on the host and may not be apparent, for example, during the incubation period or latent when little or no replication is occurring (eg, with herpesviruses). Disease represents a clinically apparent response by or injury to the host as a result of infection. With many communicable microorganisms, infection is much more common than dis-ease, and apparently healthy infected individuals play an important role in disease propa-gation. Inapparent infections are termed subclinical, and the individual is sometimes referred to as a carrier. The latter term is also applied to situations in which an infectious agent establishes itself as part of a patient’s flora or causes low-grade chronic disease af-ter an acute infection. For example, the clinically inapparent presence of S. aureus in the anterior nares is termed carriage, as is a chronic gallbladder infection with Salmonella serotype Typhi that can follow an attack of typhoid fever and result in fecal excretion of the organism for years.

    With some infectious diseases, such as measles, infection is invariably accompanied by clinical manifestations of the disease itself. These manifestations facilitate epidemio-logic control, because the existence and extent of infection in a community are readily apparent. Organisms associated with long incubation periods or high frequencies of sub-clinical infection such as human immunodeficiency virus (HIV) or hepatitis B virus may propagate and spread in a population for long periods before the extent of the problem is recognized. This makes epidemiologic control more difficult.