RELATED STORIES • Vaccines for international travel • Immunization recommendations for adults • Immunization: Why vaccines are so important to safeguarding health • Infectious diseases: How they spread, how to stop them • Immunizations for the immune-impaired • Immunization quiz: The truth behind lifesaving vaccines

Note: All links within content go to MayoClinic.com

Bacteria, viruses and other germs threaten your body every day. But when a disease-causing microorganism enters your body, your immune system mounts a defense, producing proteins called antibodies to fight off the invader. The goal of your immune system is to prevent illness by destroying the foreign invader or rendering it harmless.

Vaccines stimulate your body's immune system. To understand how vaccines work, you need to know a little bit about how your body achieves immunity.

Your body can become immune to bacteria, viruses and other germs in two ways:

• By getting a disease (natural immunity)
• Through vaccines (vaccine-induced immunity)

Whether it's natural or from vaccines, once you have immunity to a disease-causing organism, you're better protected from becoming ill.

Natural immunity
Natural immunity develops after you've been exposed to a certain organism. Your immune system puts into play a complex array of defenses to prevent you from getting sick again from that particular type of virus or bacterium.

Exposure to a foreign invader stimulates production of certain white blood cells in your body called B cells. These B cells produce plasma cells, which in turn produce antibodies designed specifically to fight that particular invader. These antibodies circulate in your body's fluids. The next time that invader enters your body, the antibodies will recognize it and destroy it. Once your body has produced a particular antibody, it rapidly produces more antibodies if needed.

In addition to the work of B cells, other white blood cells called macrophages confront and destroy foreign invaders. If your body encounters a germ that it has never been exposed to before, information about the germ is relayed to white blood cells called helper T cells. These cells aid production of other infection-fighting cells, including memory T cells.

Once you've been exposed to a specific virus or bacterium, the next time you encounter it, antibodies and memory T cells go to work. They immediately react to the organism, attacking it before disease can develop. Your immune system can recognize and effectively combat thousands of different organisms.

Vaccine-induced immunity
Vaccine-induced immunity results after you receive a vaccine. The vaccine triggers your immune system's infection-fighting ability and memory without exposure to the actual disease-producing germs. A vaccine contains a killed or weakened form or derivative of the infectious germ. When given to a healthy person, the vaccine triggers an immune response. The vaccine makes your body think that it's being invaded by a specific organism, and your immune system goes to work to destroy the invader and prevent it from infecting you again.

If you're exposed to a disease for which you've been vaccinated, the invading germs are met by antibodies that will destroy them. The immunity you develop following vaccination is similar to the immunity acquired from natural infection.

Several doses of a vaccine may be needed for a full immune response. Some people fail to achieve full immunity to the first doses of a vaccine but respond to later doses. In addition, the immunity provided by some vaccines, such as tetanus and pertussis, isn't lifelong. Because the immune response may decrease over time, you may need another dose of a vaccine (booster shot) to restore or increase your immunity.

Vaccines are prepared in several different ways. For each type, the goal is the same — to stimulate an immune response without causing disease.

• Live weakened vaccines. Some vaccines, such as those for measles, mumps, and chickenpox (varicella), use live viruses that have been weakened (attenuated). This type of vaccine results in a strong antibody response, sometimes making only one dose necessary to establish lifelong immunity.
• Inactivated vaccines. Other vaccines use killed (inactivated) bacteria or viruses. The inactivated polio vaccine is made this way. These vaccines are generally safer than live vaccines because the disease organisms can't mutate back into a disease-causing state once they've been killed.
• Toxoid vaccines. Some types of bacteria cause disease by producing toxins that invade the bloodstream. Toxoid vaccines, such as those for diphtheria and tetanus, use bacterial toxin that has been rendered harmless to provide immunity to the toxin.
• Acellular and subunit vaccines. Acellular and subunit vaccines are made by using only part of the virus or bacteria. The hepatitis and Haemophilus influenzae type b vaccines are made this way.

Currently, vaccines for nearly two dozen different diseases are licensed for use in the U.S. Twelve of these vaccines are recommended for children under the age of 2. According to the Centers for Disease Control and Prevention, widespread and persistent immunization efforts have lowered the incidence rates of several serious illnesses — including diphtheria, tetanus, measles and polio — by more than 95 percent since the beginning of the 20th century.

Yet many Americans remain underimmunized. This means they lack one or more of the recommended immunizations or didn't receive a full series of shots required for a vaccine and are therefore not fully protected.

Gaining natural immunity involves considerable risk. Diseases that otherwise are vaccine-preventable can kill or cause permanent disability, such as paralysis from polio, deafness from meningitis, liver damage from hepatitis B, or brain damage (encephalitis) from measles. Immunity from a vaccine offers protection similar to that acquired by natural infection. At the same time, vaccines rarely put individuals at risk of the serious complications of infection.

Some people believe that many of those affected during a disease outbreak are in fact the ones who were previously vaccinated. And they argue that immunity from vaccines isn't effective. It's true that vaccines aren't 100 percent protective. Most childhood vaccines are effective for 85 percent to 95 percent of recipients. During a disease outbreak, a number of vaccinated people will indeed catch the disease. However, those who were immunized usually have a less serious illness, while those not vaccinated are in the greatest danger.