Once an animal becomes infected, the primary mode of spread is then via respiratory aerosols. Other important means of spread are direct and indirect contact.
Sheep act as maintenance hosts.
Pigs act as amplifiers. Cattle act as indicators. When sheep or goats become infected with FMDv, the disease may not be diagnosed for a considerable time because signs and lesions can be very mild. During this time, however, the animals will be producing infectious aerosols, contaminating fomites, and spreading the virus by contact. An infected pig can produce a hundred million infectious doses per day.