were abandoned. It is now conceded that foot-and-mouth disease is
propagated by a specific virus and that every outbreak starts from some
preexisting outbreak.

So far investigators have been unable to identify or isolate the specific
organism causing the disease, although numerous attempts have been made to
cultivate and stain it by laboratory methods. Experiments have shown that
the virus will pass through standard germ-proof filters, thus indicating
its minute size and the reason it has not been detected by the staining
methods. The contagion may be found in the serum of the vesicles on the
mouth, feet, and udder; in the saliva, milk, and various secretions and
excretions; also in the blood during the rise of temperature.

A wide distribution of the virus and a rapid infection of a herd is the
result. Animals may be infected directly, as by licking, and in calves by
sucking, or indirectly by such things as infected manure, hay, utensils,
drinking troughs, railway cars, animal markets, barnyards, and pastures.
Human beings may carry the virus on their shoes and clothing and transmit
it on their hands when milking, since the udder is occasionally the seat of
the eruption. It may also be carried by dogs, cats, rats, chickens,
pigeons, etc. Milk in a raw state may also transmit the disease to animals
fed with it.

The observations made by some veterinarians would lead us to suppose that
the virus is quite readily destroyed. It is claimed that stables thoroughly
cleaned become safe after drying for a short time; hence, litter of all
kinds, such as manure or soiled hay and straw, may remain infective for a
longer time because they do not dry out. Other authorities maintain that
the virus is quite tenacious and may live in stables even so long as a
year. They also state that animals which have passed through the disease
may be a source of infection for several months after recovery.

Unlike most other infectious diseases, foot-and-mouth disease may
repeatedly attack the same animals. The immunity conferred by an attack is
of limited duration.

The period of incubation (that is, the time between the exposure of an
animal to infection and the development of the disease) is variable,
usually from three to six days. The disease may appear in 24 hours, or, in
exceptional cases, not for 18 days or even longer.

_Losses._--The highly contagious character of foot-and-mouth disease and
its rapid spread to practically all expose