The incubation period for plague is relatively short, usually 1-6 days. The incubation period varies slightly depending on the type of plague, from 1-2 days for pneumonic plague and septicemic plague to 9-12 days for those who have been vaccinated. Plague is a virulent infectious disease caused by Yersinia pestis and is a legal Class A infectious disease. There are many different types of plague. In addition, patients may experience shortness of breath, dryness, decreased blood pressure, and general failure. The clinical categories are glandular plague, pneumonic plague and septicemic plague, all of which have different characteristics. The main source of plague infection is plague patients. Plague patients are an important source of infection, especially those with pneumonic plague who cough up large amounts of blood, which can be spread by droplets. Rodents are plague hosts and are also the main source of infection for plague in humans.