Typically, fish diseases are caused by pathogens (agents that cause disease) as follows:
a) Parasites (Endoparasit / exto-parasite)
e) Causes such as genetic and environmental factors
i) Primary Disease
ii) Secondary disease
Occurs because of stress factors. This can weaken the fish and cause the fish to easily attacked by pathogens. Factors such as temperature, water quality and others, could easily weaken the fishes and they would not be strong enough to fight the pathogen attack.