FMD is a widely distributed and highly contagious viral disease of all cloven-footed animals in which it causes vesicles in the mouth, on the udder and on the feet. The virus occurs in seven serotypes which cause clinical signs which are indistinguishable. There is no cross-protection between serotypes, and within each serotype there is also considerable antigenic variation between the 64 sub-types. Antigenic variation is one of the most difficult problems in protecting animals by vaccination. It means that a wide range of vaccines must be available. The virus usually infects animals via the mucosal and lymphoid tissues of the pharyngeal/tonsillar region. Less frequently, infection can occur through cuts in the mouth or on the feet. Following replication at the point of entry, the virus enters the blood stream in which it may circulate for three to five days. A secondary phase of replication is initiated by blood-borne virus in glands such as lymph nodes, thyroid or adrenals. During this secondary phase the sites of vesicle development on the skin are established, leading to visible lesions. In cattle, vesicles followed; by ulcers appear on the tongue, in the mouth and sometimes on the muzzle or in the nostrils. Vesicular lesions on the feet involve the coronary band, bulbs of the heel and the interdigital space. Mortality in adult cattle is rarely more than 5%, but often is greater in calves. In pigs, mouth lesions are less prominent but feet lesions and lameness are notable. There is also depression, anorexia and loss of condition. High mortality of piglets is common. In sheep and goats, adults are not generally severely affected and the disease may go unnoticed. Sheep and goats can therefore serve as a reservoir of the virus which spreads to other species. A number of wild mammals also carry or develop the disease, and can play a significant role in its transmission, especially in Africa.