Vaccines protect against infectious diseases. In the United States, doctors recommend some vaccines (such as polio and measles vaccines) for everyone and other vaccines (such as flu or Lyme disease vaccines) only for groups of people who are likely to get the disease or to become seriously ill if they do get the disease. One such vaccine is pneumococcal vaccine. Pneumococcus is a bacterium that can cause infection in the blood, lungs, and other places in the body. Older people and people with certain chronic medical conditions, such as asthma or diabetes, are particularly likely to develop serious problems if they get pneumococcal infections. Current recommendations in the United States are to give pneumococcal vaccine to people 65 years of age and older or to younger people with chronic medical conditions. However, some expert committees have considered lowering the recommended age for pneumococcal vaccine to 50 years. Whether the costs of giving pneumococcal vaccine to people 50 through 64 years of age are worth the potential health benefits is unknown.