Disease is the result of a set on interactions amongst the host, the pathogen, and the environment. Host factors include the species of fish, size or age, strain or stock, immune status, and general physiological condition. For the pathogen, factors include the concentration of infectious particles present in the water, their physiological properties and the virulence of the strain.

The difference between apparent health and disease typically depends on the balance between the pathogen and the host, and that balance is greatly influenced by environmental factors, such as temperature and water chemistry.