Epizootic

High population density is a major contributing factor to epizootics.

The aquaculture industry is sometimes plagued by disease because of the large number of fish confined to a small area.

Defining and declaring an epizootic can be subjective; health authorities evaluate the number of new cases in a given animal population during a given period, and estimate a rate of spread that substantially exceeds what they might expect based on recent experience (i.e. a sharp elevation in the incidence rate).

Because the judgement is based on what is "expected" or thought normal, a few cases of a very rare disease (like a transmissible spongiform encephalopathy outbreak in a cervid population) might be classified as an "epizootic", while many cases of a common disease (like lymphocystis in esocids) would not.

[2][3] An example of an epizootic was the 1990 outbreak of Newcastle disease virus in double-crested cormorant colonies on the Great Lakes that resulted in the death of some 10,000 birds.