This is what causes the plague. Yersinia pestis bacteria, colored in yellow, are seen on the spines inside a flea's digestive system. National Institute of Allergy and Infectious Diseases
The bacterium that is responsible for the plague can sometimes infect the blood, causing the hands, feet, nose and lips to become gangrenous and black. This form of the disease is almost always fatal if not treated with antibiotics. CDC / Dr. Jack Poland
A map shows reported cases of human plague in the United States from 1970 to 2012. Nearly all cases occur in the western U.S., for reasons that are not entirely understood. Plague first came to the U.S. in 1900 via rats on steamships from Asia. CDC
Rats are one of the many rodent species that carry the plague. The disease is typically spread to people through a bite from a rodent flea. Although recent research may exonerate rats as primarily responsible for the Black Death, and instead put the blame on gerbils, rats probably played a role in that and other plague epidemics. Time Life Pictures/Getty Images
The plague doctor is in. The hat, goggles, gown and beak-like mask identified a person as a plague doctor in the Middle Ages. The uniform was used for protection; the beak contained herbs and perfumes intended to cover the stench associated with plague disease.
A patient with plague symptoms, foreground, awaits test results with his mother at New Delhi's Disease Hospital in 1994. Pneumonic plague, which infects the lungs, is the most serious form of the disease and the only way it can spread directly between people. A plague outbreak in India in 1994 was among the most serious in the world in recent decades. AP Photo/John Moore
Taylor Gaes died of the plague this spring, a day before he would have turned 16. Officials think he was infected from a flea bite on his family's ranch in Larimer County, Colorado. The high school sophomore began having flu-like symptoms after he pitched in a baseball game. Although plague remains rare, it is often mistaken for the flu because it causes fever and chills. From Poudre High School