Pneumonic plague is the most virulent and least common form of plague, caused by the bacterium yersinia pestis. Typically, pneumonic form is due to a secondary spread from advanced infection of an initial bubonic form.
Primary pneumonic plague results from inhalation of aerosolized infective droplets and can be transmitted from human to human without involvement of fleas or animals. Untreated pneumonic plague has a very high case-fatality ratio.
If someone inhales droplets of moisture containing plague bacteria, the result can be primary pneumonic plague. This can happen when an infected person coughs or sneezes. Cats can contract pneumonic plague, and they can transmit the disease to humans when they cough or sneeze.
As with septicemic plague, pneumonic plague can be a complication of bubonic plague -- in this case, it is known as secondary pneumonic plague. Pneumonic plague causes the typical symptoms of pneumonia, including high fever and a cough that produces bloody sputum.