Plague is a communicable disease caused by a gram-negative bacterium called Yersinia Pestis. The bacterium is transmitted from dead animals by a flea, which is the carrier of the disease. When infections occur in the lymph glands and ducts, it is referred to as the bubonic plague. If it causes a direct infection in the lungs, it is referred to as the pneumonic plague.
Pneumonic plague is a serious type of lung infection and is more dangerous than the bubonic plague. Primary pneumonic plague is an illness that results from inhaling fine droplets in the air, which can spread from one person to another. If this form of plague goes untreated, it has a 100 percent mortality rate. In secondary pneumonic plague, the pathogens obtain entry into the respiratory system from the blood.
Bubonic plagues result from the bite of the flea, which has Yersinia in its gut. After three to seven days of exposure, flu-like symptoms come and include fever, vomiting, and headache. Bubonic plagues infect the lymphatic system, which can lead to severe inflammation. If it is untreated, it can move into the blood or the lungs, causing the septicemic plague.