Type 3 secretion system of Aeromonas salmonicida is associated with immunosuppression in fish. The presence of a functional type 3 secretion system is associated with death in fish infected with Aeromonas salmonicida. Aeromonas salmonicida with a defective type 3 secretion system is not lethal but still causes immunosuppression. Infection with Aeromonas salmonicida “cured” (no T3SS and effector genes) is not lethal and causes no immunosuppression.