Classification of fish diseases is most convenient according to the etiological principle, i.e., depending on the causes that cause diseases. All fish diseases are divided into two groups - contagious and noncontagious.

Contagious diseases of fish are distinguished between infectious diseases, the causative agents of which are viruses, bacteria, fungi, and invasive diseases caused by parasites - protozoa, helminths, crustaceans, etc. Infectious diseases are subdivided into mycoses, bacterioses, rickettsioses, viral and algeoses. Infectious diseases are subdivided into mycoses, bacterioses, rickettsioses, viral and algeoses.

Contagious diseases are caused by living organisms that, intensively multiplying in the body of the affected fish, then move to healthy fish, sometimes causing their mass mortality.

Contagious diseases can be infectious: carp rubella, carp pox, furunculosis, branchiomycosis, saprolegniosis, etc., and invasive: ichthyophthyriasis, dactylogyrosis, hydrodactylosis, diplostomosis, tetracotylosis, ligulosis, argulosis, ergasilosis, etc.

The branch of science that studies fish diseases is called ichthyopathology.