Animal diseases can be caused by unicellular micro-organisms (bacteria), leading to infections and toxaemia. The virulence of bacterial animal diseases depends on the degree of immunity of the host. There are four main types of bacteria: the spherical or coccus form; the rod or bacillus type; the spirally twisted spirillum; and a long filamentous type. The extent of the disease is dependent on the ability of the bacteria to multiply in the host. Transmission may be by contagion, inhalation, infection from food, via insects, or via wounds. Bacterial animal diseases include plague, glanders pseudotuberculosis, enteritis, mastitis, pneumonia, infectious anaemia, anthrax, food poisoning, tuberculosis, Johne's disease, meningitis and haemorrhagic septicaemia.