Different strains of the same pathogen may vary considerably in their ability to cause disease. That is, some strains can infect a host without causing a clinical disease. Also, individual animals differ in susceptibility to disease. Some individuals may be more resistant to the disease because of their genetic composition or as a result of previous exposure and development of immunity. Conversely, individuals may be more susceptible to a disease because of poor nutritional status, stress or any of a number of other factors.
Population density is a particularly important factor in the spread of fish diseases.
Diseases can spread more easily within dense populations simply because of increased opportunities for association of infected fish with uninfected fish. The actual development of a disease and the relevant severity of that disease within a fish population is influenced by a complex interaction of these variables associated with the pathogen, the host and the environment. Diseases in Aquaculture Populations . . . Infectious diseases pose one of the most significant threats to successful aquaculture. The maintenance of large numbers of fish crowded together in a small area provides an environment conducive for the development and spread of infectious diseases.