AGf~ICULTURAL RESEARCH, PUSA.

462 PRACTICAL BACTERIOLOGY
Foot-and-mouth disease is another example; the
difficulty of control is due to the amazing rapidity of
spread, not only from animal to animal, but from
one locality to another.
As a group, these viruses are markedly pathogenic,
even in minute doses. The virus of yellow fever is
capable of cnusing il1feetion of the monkcy in a
quantity as small as 0·000001 C.c. of an emulsion
of brain tissue from an infected animal. Incredibly
small amounts of these viruses are capable of
causing infection, as is shown by the rapid spread
of the disease in spite of the utmost attempts at
control.
2. TIm PnoDucTION Oli' IMMUNI'l'Y.-In individuals
surviving infection t.herc is in many of t.he diseases
a high degree of immunit.y, which usually lasts for long
periods of time. Advantage is taken of this in immunisation
against smallpox and rabies by inoculation
with attenuated viruses. In addition, specific antisera
may be prepared, as in the case of hog-cholera and
cattle plague, which arc available for specific treatment.
On the other hand, the immunity following infection
may be of only short duration.
Prolonged Irmll.l(llity
Smallpox.
Vaccinia.
Chiekenpox.
Measles.
Yellow fever.
Rift Valley fever.
Mumps.
.