Like all animals, wild and farmed fish and shellfish experience disease problems. These may be broadly classified into infectious and non-infectious diseases. However, it is not always easy to make a strict division between these as non-infectious causes can stress fish and render them more susceptible to disease agents such as bacteria and viruses.
Fish and shellfish, unlike mammals, are cold-blooded animals and all their physiological processes, including their ability to mount an immune response to pathogens, are greatly influenced by water temperature. Similarly, the pathogens which affect fish are also influenced by water temperatures which means that diseases and diseases outbreaks are often seasonal in their occurrence which varies between diseases, some occurring during the colder winter months, with others predominantly occurring during the warm summer months.