Lung Disorder Pneumonia is an infection of the lungs with a range of possible causes. It can be a serious and life-threatening disease. It normally starts with a bacterial, viral, or fungal infection. The lungs become inflamed, and the tiny air sacs, or alveoli, inside the lungs fill up with fluid. Here are some key points about this disease. More detail is in the main article. Pneumonia is an infection of the lungs that can cause mild to severe illness in people of all ages. It is the leading cause of death due to infection in children younger than 5 years of age worldwide. Pneumonia and influenza together are ranked as the eighth leading cause of death in the U.S. Those at high risk for this disease include older adults, the very young, and people with underlying health problems. Cause of Pneumonia Bacteria and viruses are the main causes of this disease. This disease-causing germs can settle in the alveoli and multiply after a person breathes them in. Pneumonia can be contagious. The bacteria and viruses that cause pneumonia are usually inhaled. They can be passed on through coughing and sneezing or spread onto shared objects through touch. The body sends white blood cells to attack the infection. This is why the air sacs become inflamed. The bacteria and viruses fill the lung sacs with fluid and pus, causing pneumonia. Risk Those most at risk include people who: Are aged under 5 years or over 65 years Smoke tobacco, consume large amounts of alcohol, or both have underlying conditions such as cystic fibrosis, chronic obstructive pulmonary disorder (COPD), asthma, or conditions that affect the kidneys, heart, or liver have a weakened or impaired immune system, due, for example, to AIDS, HIV, or cancer take medicines for gastroesophageal reflux disease (GERD) have recently recovered from a cold or influenza infection experience malnutrition have been recently hospitalized in an intensive care unit have been exposed to certain chemicals or pollutants