Immunization enables the body to better defend itself against diseases caused by certain bacteria or viruses. Immunity may occur naturally (when people are exposed to bacteria or viruses), or doctors may provide it through vaccination. When people are immunized against a disease, they usually do not get the disease or get only a mild form of the disease. However, because no vaccine is 100% effective, some people who have been immunized still may get the disease.
In communities and countries where vaccines are widely used, many diseases that were once common and/or fatal (such as polio, measles, and diphtheria) are now rare or under control. One disease, smallpox, has been completely eliminated by vaccination. Vaccines have been very effective in preventing serious disease and in improving health worldwide. However, effective vaccines are not yet available for many important infections, including Ebola virus infection, most sexually transmitted diseases (such as HIV, syphilis, gonorrhea, and chlamydial infections), and many tropical diseases (such as malaria).
Following recommendations for vaccination is very important for people's own health and for the health of their family and the people in their community. Many of the diseases prevented by vaccination are still present in the United States and remain common in other parts of the world. These diseases can spread rapidly among unvaccinated children, who, because of the ease of modern travel, can be exposed even if they live in areas where a disease is not common.
Vaccines available today are highly reliable, and most people tolerate them well. They rarely have side effects.