Illness, in fish as well as in other animals, when caused by an infectious disease, is often not the result of infection with pathogens alone. In many cases the pathogens and hosts can exist side by side without development of disease symptoms. Such symptoms, with resulting illness or death, appear only when the balance of mutual tolerance between the host and the pathogen is shifted in favor of the pathogen. Factors of several different kinds may render a host susceptible to disease; such things as poor nutrition, unfavorable physical conditions in the environment and the hereditary make-up of the fish itself. Drugs may allow temporary control of infectious diseases but for the long-range control it is better to increase the resistance of the host.
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