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.
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.