Pneumonia is sneaky, to say the least. It often strikes in the midst of another illness, like bronchitis or the flu. But it's a lot less of a death sentence than it used to be. In 1900, it was the No. 1 cause of death in the U.S. In 2006, pneumonia was the eighth most common cause of death, along with influenza [sources: American Lung Association and CDC].
Caused by fungi, bacteria, viruses or other small germs, pneumonia occurs when said germs reach and infect one or both lungs [source: American Lung Association]. Although it is typically treatable, pneumonia is sometimes difficult to get under control, and can cause difficulty breathing, lung abscess and other complications. Bacteria can also invade the bloodstream, potentially causing organ failure [source: Mayo Clinic]. Experts typically advise high-risk groups, particularly people over age 65, to be vaccinated to prevent getting sick or experiencing complications [source: CDC].