Usually associated with the massive waves of "black death" that swept across Europe during the Middle Ages, the plague still occurs occasionally in the United States, though it is treatable with modern antibiotics.
The disease, caused by a bacterium called Yersinia pestis, most often is transmitted to humans through rodent flea bites or by handling infected animals.
Weaponizing the Plague
Experts are skeptical that the plague would make a good bioterror agent.
"It is possible, but difficult, to effectively weaponize Y. pestis," said John D. Clements, professor and chair of the program in molecular pathogenesis and immunology at the Tulane University School of Medicine in New Orleans.
"The only effective way would be to aerosolize the organisms and this would be much more difficult than for anthrax," he added. "This is mostly due to the fact that, unlike Bacillus anthracis, Yersinia do not make spores. Keeping the organism suspended at a high enough concentration, at the right particle size and viable is problematic."
However, a fact sheet on the U.S. Centers for Disease Control and Prevention's Web site notes that if someone with "advanced knowledge and technology" were somehow able to aerosolize the bacterium, it could cause damage.
"Yersinia pestis used in an aerosol attack could cause cases of the pneumonic form of plague," the CDC says. "One to six days after becoming infected with the bacteria, people would develop pneumonic plague. Once people have the disease, the bacteria can spread to others who have close contact with them. Because of the delay between being exposed to the bacteria and becoming sick, people could travel over a large area before becoming contagious and possibly infecting others. Controlling the disease would then be more difficult."
Pneumonic plague differs from bubonic plague in its symptoms and the fact that it can be spread through the air rather than just by contact.
Unlike bubonic plague, pneumonic plague can be spread from person to person. According to the CDC, "Pneumonic plague affects the lungs and is transmitted when a person breathes in Y. pestis particles in the air."
Occurs in the Wild
If untreated, people still can die from the plague, which in the United States occurs in the wild, primarily in rural parts of western states, at a rate of about 10 to 15 cases per year, according to the CDC. Most of the naturally occurring cases are bubonic plague, which can bring on pneumonic plague if left untreated and a person's lungs become infected.
However, the disease is completely treatable with modern antibiotics if it is diagnosed early.
Worldwide, the World Health Organization reports 1,000 to 3,000 cases of plague each year.
Most infections in the United States have occurred after disposing of squirrels or mice that died from the infection or traveling in an area where infected rodents live. Health officials recommend staying away animals that are lethargic or appear sick.
The bacteria that causes bubonic plague is spread by infected fleas to rodents, most commonly mice, ground squirrels, rock squirrels, prairie dogs and even cats and dogs.
Early symptoms of bubonic plague infection include swollen lymph nodes, fever, chills, headache, muscle pain, nausea, vomiting and diarrhea. These signs will usually appear two to seven days after infection.
State health departments and the CDC regularly test rodent populations to track the spread of plague bacteria in the wild. If a large population of infected rodents is found, they will spray insecticide at the area to kill fleas, and post signs warning residents to stay away from the area.
Health officials say the bacteria don't pose serious risk to American cities because it is isolated to rural areas and usually stays there.
The last urban outbreak of the plague in the United States occurred in Los Angeles in 1924 to 1925, the CDC says.