The environment in which an animal lives affects the physiology and psychology of that animal. The greater the distance from the equator the more profound this influence becomes, as the environment becomes more variable over the years. Temperature, photoperiod, precipitation and other environmental conditions, which are directly or indirectly controlled by the season, can affect an animal. It is becoming apparent that these conditions may impact on the immune system, and this can affect animal health. This review looks at the known mechanisms for transducing environmental cues and how these can affect immune parameters and function. The main focus is fish, especially in relation to aquaculture and the associated disease risks. Work on other animal classes is included for comparison.