The Plague, often thought of as just one disease, is actually a term that
refers to several systemic diseases caused by the bacteria Yersinia Pestis. Yersinia
pestis is a bacterium that is primarily seen in rodent populations, and more
specifically in rats. It is a member of the
family of bacteria,
and is Gram negative when looked at under a microscope.
Some other examples
include salmonella and shigella. Plague is exemplified by
periodic outbreaks of disease in the rodent populations. During these outbreaks,
infected fleas that have been deprived of their normal hosts search for other
sources of blood. This increases the risk to humans and other animals frequenting
the area because they are the next source of blood sought out by the fleas.
humans are infected with the bacteria, the resulting disease is known as the
Bubonic plague, named for the “buboes” or swollen glands that it causes. There
are also two other manifestations of the plague named the septicemic plague and
the pneumonic plague. Epidemics of plague in humans usually involve house rats
and the fleas that live on them. These epidemics continue to occur in some
developing countries, particularly in rural areas where house rats are more
Transmission between humans can also occur with the pneumonic
plague because it can be spread in air droplets produced from the human body,
usually through a cough or sneeze that sprays the droplets into the air.
There are various signs and symptoms that indicate an infection with the
bubonic, pneumonic, or septicemic plagues. If the vector is a flea, the bacterium
lingers in its esophagus, preventing food intake, which causes the flea to
desperately seek food. When it bites the human, it regurgitates bacterium laden
fluid beneath the skin, where it begins to invade the surrounding cells, marking the
beginning of an infection.
The infection attacks nearby lymphoid tissue,
producing the namesake bubo, an inflamed, virulent, and hemorrhagic lymph
The infection can potentially spread to every organ, including the lungs,
kidneys, liver, spleen and rarely even the brain. The more virulent pneumonic
plague is spread by direct contact and inhalation of the bacteria, or when the
bubonic plague spreads into the airways and lungs. The septicemic plague occurs
when the infection proceeds directly to the bloodstream as opposed to passing
through the lymphatic system first. Because of this, the bubo does not occur in
The range of symptoms for the plague is wide, and differs depending on the
manifestation of the infection in the body.
All three manifestations of plague are
acute, meaning that the symptoms appear suddenly after the bacteria invades the
body, making accurate diagnosis and quick treatment vital to the survival of the
"Plague Information." Division of Vector-Borne Infectious Diseases
. 30 Mar 2005. Centers for Disease
Control. 9 Dec 2007 <http://www.cdc.gov/ncidod/dvbid/plague/info.htm>.