Pneumonia is a common illness that affects millions of people each year in the United States. Pneumonia is an infection of the lungs. Many different organisms can cause it, including bacteria, viruses, and fungi. Pneumonia can range from mild to severe, and can even be deadly.

The severity depends on the type of organism causing pneumonia, as well as your age and underlying health. Bacterial pneumonias tend to be the most serious and, in adults, the most common cause of pneumonia. The most common pneumonia-causing bacterium in adults is Streptococcus pneumoniae (pneumococcus). Respiratory viruses are the most common causes of pneumonia in young children, peaking between the ages of 2 and 3. By school age, the bacterium Mycoplasma pneumoniae becomes more common.

In some people, particularly the elderly and those who are debilitated, bacterial pneumonia may follow influenza or even a common cold. Many people contract pneumonia while staying in a hospital for other conditions. This tends to be more serious because the patient's immune system is often impaired due to the condition that initially required treatment. In addition, there is a greater possibility of infection with bacteria that are resistant to antibiotics.

The main symptoms of pneumonia are:
-- Cough with greenish or yellow mucus; bloody sputum happens on occasion
-- Fever with shaking chills
-- Sharp or stabbing chest pain worsened by deep breathing or coughing
-- Rapid, shallow breathing 
-- Shortness of breath
-- Additional symptoms include:
   Headache 
   Excessive sweating and clammy skin
   Loss of appetite
   Excessive fatigue
   Confusion , especially in older people

Other types of pneumonia:
Aspiration pneumonia
Atypical pneumonia
CMV pneumonia
Hospital-acquired pneumonia
Legionella pneumonia
Pneumocystis carinii pneumonia
Pneumonia in immunocompromised host
Viral pneumonia
Walking pneumonia (Atypical pneumonia)

Source:  National Institutes of Health; National Library of Medicine