One of the primary concerns of any aquaculture program is the potential introduction
and transmission of pathogens in both cultured and native populations. Exposure to
infectious disease agents is a continual process during the life span of any organism.
However, exposure to an infectious microorganism does not necessarily result in infection
or manifestation of clinical disease. The latter depends on the interaction of several factors
including (1) the health and immunological status of the host, (2) the dose and virulence or
contagiousness of the pathogen, and (3) the environmental conditions that affect the host
and pathogen interaction. Although clinical disease is easily qualified and quantified,
subclinical disease is more difficult to characterize and may only be detected with the
assistance of diagnostic tests or aids. However, it must be emphasized that the presence or
detection of any infectious agent does not imply the presence of disease. Simply put,
infection—defined as invasion of a host by a pathogenic agent—is a more common event.
In contrast, disease is defined as the condition that results in morbidity and, possibly,
mortality in the individual host or population as a consequence of infection.