Epizootic animal diseases affect many animals in a large area at the same time, and spread with great rapidity. Epizootic diseases are generally contagious from one animal to another but may be spread by intermediate agents such as forage, straw, insects, fertilizers, contaminated water, movements of game or wild animals and bird migration, or sea seasonal factors. Insect-borne diseases have a higher propensity to become epizootic, especially when the insects are numerous, such as with flies and trypanosomiasis in Africa.