Everybody can be stressed including aquacultured animals
Stress is a challenge for any organism. Although the idea of stress as a pathological situation is mostly associated with mental status and anxiety in man, the same situation can be applied to most living organisms. Therefore, the responses to an acute stressor (flying, fighting, fear) are applicable to animals including fish and so is the response to chronic stress, causing energetic and immune suppression, and therefore allowing the infectious agents to enter the organism and produce disease.

Fish and shellfish have more powerful non-specific immune defence
During the last years it has become more and more evident that lower vertebrates and specifically fish have potent non-specific or innate immune responses and less potent specific or adaptive immune defences. For instance, one of the main non-specific responses, the fish complement system has 5-10 times more powerful a response with a much higher versatility than humans, whereas the immunoglobulin (specific) activity is less versatile, slower and less potent than in humans. That is the reason why future therapy for fish will be based on these innate responses.

Most of fish health treatments are performed through the feeds
When a vaccine or a chemical treatment may be available for use in fish it is necessary for a fish farmer or a veterinary to know whether it will be worth delivering this to the fish. For large numbers of fish and depending on the size it may be impossible to deliver the vaccine by bath treatment (as the quantity of the agent may be unaffordable) or by injection (due to high numbers and the stress given to the animal). Therefore, most health treatments are performed via inclusion of the therapeutic agent into the food. This means that feed technologists and pharmaceutics have to deal with aspects like digestibility, degradation-effectiveness of the active principle combined with other feed ingredients.