10 Deadly Agents the CDC Works With

You’re looking at the Oriental rat flea (Xenopsylla cheopsis), a primary vector for bubonic plague. © Dr. Robert Calentine/Visuals Unlimited/Corbis

Our next contestant needs no introduction. Best known for killing 60 percent of Europe in the 14th century, it has helped to topple empires and drive social and technological change. Today, you can find it mainly lying low in small towns or villages in developing areas of Africa, Asia and South America, but it also enjoys the American West, where an average of seven cases per year popped up between 1900 and 2010 [source: CDC].

The Yersinia pestis bacteria cause three kinds of plague:

  1. Bubonic plague, typically contracted from the bite of infected rodent fleas
  2. Septicemic plague, contracted through flea bites, exposure to the fluids or remains of plague-infected animals, or from untreated bubonic plague
  3. Pneumonic plague, which arises from inhaling respiratory droplets from infected cats or humans that have pneumonic plague or other plague that has spread to the lungs

Bubonic sufferers quickly develop fever, headache, chills, weakness and enlarged, painful lymph nodes called buboes, where the plague bacteria multiply. Unless treated with the right antibiotics, this form of plague can spread throughout the body and develop into the other types. Septicemic patients add symptoms of abdominal pain, shock and possible internal bleeding, while pneumonic types exhibit a quickly worsening pneumonia that can result in respiratory failure and shock.

Thanks to antibiotics, plague's mortality rate has dropped from 66 to 11 percent. Untreated, septicemic and pneumonic plagues consistently kill their victims [sources: CDC; WHO].