The viruses and bacteria that cause pneumonia are contagious and are usually found in fluid from the mouth or nose of an infected person. Illness can spread through exposure to coughs and sneezes, shared drinking glasses and eating utensils, and used tissues or handkerchiefs. People with respiratory disease and weakened immune systems are at greatest risk. Pneumonia bacteria sometimes live in healthy throats without causing illness. If, however, the body's defenses are weakened by disease, malnutrition, impaired immunity, or other conditions, the bacteria can start to multiply.
Infection then spreads quickly through the bloodstream.
Symptoms of bacterial pneumonia may occur slowly or very suddenly. In the most severe cases, bacterial pneumonia causes shaking chills, severe chest pain, and a cough that yields rust-colored or greenish mucus. Sometimes lips and nailbeds look blue because of lack of oxygen. Viral pneumonia About half of all pneumonias are believed to be caused by viruses. The viruses usually attack the nose and throat, but some also infect the lungs, especially in children.