Invasive diseases are caused by protozoa (ciliated infusoria, flagellates, sporozoa), various parasitic worms (trematodes, cestodes, nematodes, etc.), parasitic crustaceans. The most widespread are ichthyophthyriosis, dactylogyrosis, diplostomosis, ligulosis, argulosis, ergasilosis and others.

The naming of these diseases is currently carried out in full compliance with the nomenclature of invasions developed in 1928 by scientists K. I. Skryabin and R. S. Shchultz, which is based on a zoological basis. It was recommended to name diseases by the zoological name of the genus of the causative agent with the addition of the suffix "oz" or "ez" to the root of the word. For example, the genus Costia is the disease Costiasis, the genus Ligula is the disease Ligulosis.

The causative agents of invasive fish diseases can be protozoan organisms, parasitic worms, parasitic arthropod crustaceans and molluscs. Depending on this, in ichthyopathology, invasive diseases are classified into the following groups of pathogens: protozooses, celenteratoses, helminthoses, crucetaceoses and molluscoses.