There are three main forms of plague in humans, namely bubonic, septicaemic and pneumonic. Bubonic plague is the result of an insect bite in which the plague bacillus travels through the lymphatic system to the nearest lymph node where it forms a swelling. The most usual place for this swelling is the groin, but it also occurs in the armpits and the neck. The septicaemic form of plague occurs when the insect deposits the bacillus directly into the bloodstream. This form is almost always fatal.
Pneumonic plague is an infection of the lungs with the plague bacillus.
Pneumonic plague can be transmitted directly from person to person via infected air droplets or through infected clothing and other contaminated articles. Plague has declined dramatically since the early part of the twentieth century, when outbreaks could cause tens of millions of deaths. This is due primarily to improvements in living standards and health services. However, a substantial number of countries continue to be affected by plague, case fatality rates remain high and antimicrobial resistance has begun. Therefore, continued vigilance is required, particularly in human populations living near natural plague foci.