i) Virus replication is rapid and the animal is dead before an immune response can be made e.g. Fowl plague or the virus outstrips the immune response e.g. acute ASFV, SFV .

ii) Irreversible virus necrosis in an essential organ e.g. the brain e.g. Rabies.

iii) Immune complex disease e.g. FIP, VM.

iv) Immunosuppressive virus e.g. FIV, PRRS.

v) Latency, virus escapes immunity, e.g. Herpesviruses.

vi) Antigen-specific tolerance e.g. ALV, BIV, BVDV.