Often called hoof-and-mouth disease, this highly contagious virus causes blisters in the mouth and on the hoofs of animals with cleft, or divided hoofs, such as sheep, cattle and hogs. The disease was first noted in Europe in 1809; the first outbreak in the United States came in 1870. Although it seldom spreads to humans, it can be transmitted through contaminated milk or the handling of infected animals.

Outbreaks are expensive for the animal owners who must kill the infected animals and bury them in quicklime. Then the animals' living quarters are disinfected, while feed and litter are burned. The farm is quarantined by state and federal officials who can decide to extend the quarantine to the general area or the whole state. Friedrich August Löffler (1852-1915), a German bacteriologist who discovered the bacillus of diphtheria in 1884, also demonstrated in 1898 that a tiny virus causes hoof-and-mouth disease. It was the first time a virus was reported to be the cause of an animal disease.

An infected animal can take up to four days to begin showing symptoms of fever, smacking of lips and drooling. Eventually, blisters appear on the mouth, tongue and inside of the lips, and the animal becomes lame just before blisters appear in the hoof area.

Löffler, working with Dr. Paul Frosch (1860-1928), a veterinary bacteriologist, extracted lymph from the blisters on the mouths and udders of diseased cattle. The lymph was diluted in sterile water and passed through filters. The researchers expected the filtrate to be an antitoxin of foot-and-mouth disease similar to the one for smallpox.

But Löffler and Frosch were wrong; when the filtrates were injected into healthy animals, they became sick. Therefore, they concluded the causative agent was not a bacterial toxin, but instead was a non-toxin producing bacterium too small to be seen under the microscope, yet small enough to pass through the filters. It wasn't until 1957 that scientists were able to get their first look at the causative agent--one of the smallest viruses to cause an animal disease. Scientists now know that this virus has several different strains and numerous sub-variants. Extremely contagious, the virus spreads easily, including via air currents, clothing, animal feeds, and human contact.