Vaccines help a body prepare in advance to fight illnesses and potentially deadly diseases. Essentially, vaccines give the body a preview of a bacterium, virus, or toxin, allowing it to learn how to defend itself in advance. If the body is ever invaded by that particular pathogen after the vaccine has done its work, the body’s immune system is ready.

Most vaccines are administered in the form of a shot or a liquid that is consumed by mouth. However, some vaccines are inhaled as aerosols or powders. The majority of vaccines contain viruses or bacteria that have been weakened or killed. Others contain inactivated toxins. In their altered states, vaccine pathogens are typically safe and unable to cause disease.

When a weakened or dead pathogen is introduced into the bloodstream, the body’s B-cells go to work. It is these cells that are responsible for fighting disease-causing pathogens. Once the B-cells are stimulated to act, antibodies are formed and the body develops immunity to the particular pathogen. Once a person receives a vaccine and develops immunity, he or she is usually protected for life.

Sometimes vaccines do not provide lifetime immunity. For example, some vaccines, such as tetanus and pertussis, are only effective for a limited amount of time. In such cases, booster shots are necessary to maintain continuous vaccine protection. These booster doses are given at specific intervals following the initial vaccination.

One vaccine, the flu vaccine, must be given every year. This is because there are many strains of the flu. A vaccine given in one year may provide protection against certain strains of the flu virus, but when the next year’s flu season starts, vaccination against newer or different strains may be required. Furthermore, flu vaccines do not provide protection for a lifetime. After as little as one year, protection may be far less than adequate.

Most human beings are born with a limited immunity to a variety of diseases. This immunity is the result of antibodies that are passed on to infants by their mothers. Unfortunately, these antibodies are only capable of protecting infants for about a year after birth. Additionally, there are many serious diseases for which infants do not receive antibodies from their mothers. To protect human beings from infancy into adulthood, many vaccines are given in the first months of life.