Plague is transmitted from animal to animal and from animal to
human by the bites of infective fleas. Less frequently, the organism
enters through a break in the skin by direct contact with tissue
or body fluids of a plague-infected animal, for instance, in the
process of skinning a rabbit or other animal. Plague is also transmitted
by inhaling infected droplets expelled by coughing, by a person
or animal, especially domestic cats, with pneumonic plague. Transmission
of plague from person to person is uncommon and has not been observed
in India since 1995 but does occur as an important factor in plague
epidemics in some developing countries.
Fleas become infected by feeding on rodents, such as the chipmunks,
prairie dogs, ground squirrels, mice, and other mammals that are
infected with the bacteria Yersinia pestis. Fleas transmit the
plague bacteria to humans and other mammals during the feeding
process. The plague bacteria are maintained in the blood systems
of rodents.
