Plague
The infection: Plague, caused by the bacteria Yersinia pestis.
Symptoms and treatment: Symptoms vary on the form of infection it takes. The most serious kind is pneumonic plague, which is highly deadly without prompt antibiotic treatment and can be contagious to others.
Threat potential: Plague is responsible for some of the worst pandemics in human history. The availability of antibiotics and improved sanitation (its most common vectors are rodents and their fleas) have made it less of a threat these days, but it still causes periodic outbreaks. Plague bacteria circulate in several animal hosts, but some recent research has suggested that it hides inside soil-loving amoeba as well. And at least some scientists believe that its disappearing act in soil is how the plague has managed to reemerge to wreak havoc time and time again. Climate change may also allow it to become a more common threat in the future.
