Vaccination is the use of vaccines to prevent specific diseases.

Many diseases that once caused widespread illness, disability, and death nowcan be prevented through the use of vaccines. Vaccines are medicines that contain weakened or dead bacteria or viruses. When a person takes a vaccine, hisor her immune system responds by producing antibodies--substances that weaken or destroy disease-causing organisms. When the person is later exposed to live bacteria or viruses of the same kind that were in the vaccine, the antibodies prevent those organisms from making the person sick. In other words, theperson becomes immune to the disease the organisms normally cause. The process of building up immunity by taking a vaccine is called immunization.

Vaccines are used in several ways. Some, such as the rabies vaccine, are given only when a person is likely to have been exposed to the virus that causesthe disease--through a dog bite, for example. Others are given to travelers planning to visit countries where certain diseases are common. Vaccines such as the influenza vaccine, or "flu shot," are given mainly to specific groups of people--older adults and others who are at high risk of developing influenza or its complications. Then, there are vaccines that are given to almost everyone, such as the one that prevents diphtheria.

Children routinely have a series of vaccinations that begins at birth. Givenaccording to a specific schedule, these vaccinations protect against hepatitis B, diphtheria, tetanus, pertussis (whooping cough), measles, mumps, rubella(German measles), varicella (chickenpox), polio, and Haemophilus influenzae type b (Hib disease, a major cause of spinal meningitis). This seriesof vaccinations is recommended by the American Academy of Family Physicians,the American Academy of Pediatrics, and the Centers for Disease Control and Prevention and is required in all states before children can enter school.

In addition, vaccines are available for preventing anthrax, cholera, hepatitis A, Japanese encephalitis, meningococcal meningitis, plague, pneumococcal infection (meningitis, pneumonia), tuberculosis, typhoid fever, and yellow fever. Most vaccines are given as injections, but a few (such as the oral polio vaccine) are given by mouth.

Some vaccines are combined in one injection, such as the measles-mumps-rubella (MMR) or diphtheria-pertussis-tetanus (DPT) combinations.

Anyone planning a trip to another country should check to find out what vaccinations are needed. Some vaccinations must be given as much as 12 weeks before the trip, so getting this information early is important. Many major hospitals and medical centers have travel clinics that can provide this information. The Traveler's Health Section of the Centers for Disease Control and Prevention also has information on vaccination requirements.

Vaccines are not always effective, and there is no way to predict whether a vaccine will "take" in any particular person. To be most effective, vaccination programs depend on whole communities participating. The more people who arevaccinated, the lower everyone's risk of being exposed to a disease. Even people who do not develop immunity through vaccination are safer when their friends, neighbors, children, and coworkers are immunized.

Like most medical procedures, vaccination has risks as well as substantial benefits. Anyone who takes a vaccine should make sure that he or she is fully informed about both the benefits and the risks. Any questions or concerns should be discussed with a physician or other health care provider. The Centers for Disease Control and Prevention, located in Atlanta, Georgia, also is a good source of information.

Vaccines may cause problems for people with certain allergies. In general, anyone who has had an unusual reaction to a vaccine in the past should let hisor her physician know before taking the same kind of vaccine again. The physician also should be told about any allergies to foods, medicines, preservatives, or other substances.

Anyone who takes a vaccine should let the physician know all other medicineshe or she is taking and should ask whether the possible interactions could interfere with the effects of the vaccine or the other medicines.

People with certain medical conditions should be cautious about taking vaccines. Influenza vaccine, for example, may reactivate Guillain-Barré syndrome (GBS) in people who have had it before. This vaccine also may worsen illnesses that involve the lungs, such as bronchitis or pneumonia. Vaccines thatcause fever as a side effect may trigger seizures in people who have a history of seizures caused by fever.

Certain vaccines are not recommended for use during pregnancy, but some may be given to women at especially high risk of getting a specific disease such as polio. Vaccines also may be given to pregnant women to prevent medical problems in their babies. For example, vaccinating a pregnant woman with tetanustoxoid can prevent her baby from getting tetanus at birth. Women who are breastfeeding should check with their physicians before taking any vaccine.

Most side effects from vaccines are minor and easily treated. The most commonare pain, redness, and swelling at the site of the injection. Anyone who hasan unusual reaction after receiving a vaccine should get in touch with a physician right away.

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