Yersinia plague is caused by the bacteria Yersinia pestis
(see photo). Y. pestis
is a rod-shaped bacteria that has developed a solid reservoir among wild rodent populations, including prairie dogs, squirrels, and chipmunks but most famously associated with rats. In fact, it is speculated that the plague originated in Egypt and was introduced to the rest of the world by the black rat stow-aways on trade ships. It is very rare indeed for a human to be infected directly by a rodent, however. Much more commonly, the fleas that pester the infected rats spread the disease among humans. Y. pestis
closes off the throat of the fleas, making them unable to swallow food. The poor starving parasites go on a feeding frenzy and with each bite and unsuccessful suck, spread the bacteria into their intended snacks - other rodents, wild animals, and humans.
The most famous form of Yersinia plague, the bubonic plague, is also the least fatal of the three forms. Initial symptoms include the development of large, painful "bubos" (see photo below). The bubos are actually very swollen lymph nodes which served as points of initial infection and most often show up in the groin, under-arm, or neck. They tend to be red with a bruise around them and the tissue may die, turning the characteristic black color. Other symptoms make the common flu look like a kiddy ride - high fever, nausea, vomitting (possibly bloody), severe muscle/joint pain, sore throat, headache, debilitating weakness, chills, and a general sense of feeling so miserable you'd gladly lay down in front of a steam roller for a little relief. With modern antibiotics, if given quickly, the relief is more likely - only 15% of patients treated with antibiotic therapy and supportive therapy die, as opposed to 40-60% of people who go untreated. Bubonic plague can lead to septicemic or pneumonic plague.
Septicemia is a severe, generalized infection - the bacteria circulate through the blood stream and can impact any organ in the body. Septicemic plague can occur after the formation of bubos or without bubos (rare) and the symptoms listed under the bubonic form of the disease. In addition, septicemic plague can cause symptoms throughout the body depending on which areas are affected - diarrhea (often bloody), constipation, severe belly pain, cough (often bloody), muscle pain, stiff neck, bleeding from just about anywhere, gangrene of the fingers, toes, penis, or nose, seizures, confusion, delirium, or coma. Untreated, septicemic plague is 100% fatal and can lead to pneumonic plague.
Pneumonic plague can be contracted two different ways - from the advancement of bubonic or septicemic plague or from coming into contact with another person or animal who has pneumonic plague. This form of the disease may include bubos and bloody cough, along with general symptoms of the plague and signs of pneumonia. The difference here is unlike bubonic plague and septicemic plague, pneumonic plague is extremely contagious person-to-person. While the other forms of Yersinia plague can pass person-to-person with close contact and exchange of bodily fluid, stepping within a few feet of a person suffering pneumonic plague and taking a few unprotected breaths can be quite literally a death sentence. Survival if treated with modern antibiotics within the first 24 hours of infection with pneumonic plague is often effective at preventing death, but left untreated, this form of the plague is 100% fatal.
The pandemics that swept through the pre-modern world drastically altered the face of the world, not just in terms of the depopulation, but in political, scientific, and religious terms as well. Volumes have been written which point to the Black Death of 14th century Europe as the single most important disease event in shaping the face of the modern world, creating the infancy of modern medicine and ending the dark ages.
Modern sanitation, pest control, and antibiotics have reduced this one-time mega killer to a smaller stature on the scale of world threats, but modern man might yet feel the real bite of this beast. In the age of terrorism, Yersinia plague in aerosol (airborn) form is considered one of the most feared as a potential biological weapon. And unlike small pox and polio, this deadly disease has host colonies the world over and will very likely continue to be a threat looming over us forever.