Before vaccines were invented the only way to create immunity in the body was to suffer through a bout of the disease in question. Once endured, providing you survived, your immune system could fight off any future infections before they took hold. B-cells in the bloodstream, responsible for fighting off the disease, retain memory of the disease. If the disease returned, the immune system launched a quick attack before the disease could take hold.

Vaccines produce the same effect without making the patient suffer through the disease. By introducing a disease into the bloodstream, B-cells are stimulated into action, creating antibodies and a memory record of the pathogen, resulting in immunity.

Vaccines are safe because the viruses or bacterias used in vaccines have been severely compromised in a laboratory by one of several strategies.

In the case of measles, mumps and chickenpox, the virus's ability to reproduce rapidly is weakened. Normally these viruses cause illness by reproducing themselves thousands of times in the body. The virus used in a vaccine may reproduce itself 20 times or less, but this is still enough to initiate B-cell reaction and cell memory. One or two vaccines of a live, weakened virus will make a person immune for life. Unfortunately this type of vaccine is not safe for people with compromised immune systems, such as those suffering with cancer or AIDS.

Another strategy is to deactivate a virus by killing it with a chemical. Now the virus cannot reproduce at all, yet the presence of the dead virus in the body still generates a response by B-cells, producing antibodies and a memory record. However, as safe as this method is for people with weakened immune systems, its drawback is that it takes several vaccines to achieve long-lasting immunity. Vaccines of this type include polio, influenza, hepatitis A, and rabies.

For hepatitis B only a part of the virus is used -- a protein taken from the surface of the virus. In this case B-cells respond to the protein, immunizing the body against the entire virus. This vaccine is safe for people with weakened immune systems but requires three doses for lifetime immunity.

Several bacterial diseases have vaccines based on using part of the bacteria as well. These bacteria make harmful proteins called toxins. The toxins can be inactivated with a chemical. Once the toxin is inactivated, the bacteria is harmless, and therefore so is the vaccine. Two examples of this type of vaccine are diphtheria and tetanus.

But the most familiar vaccine of all is the flu vaccine.

Many people get the flu shot every year, which contains dead influenza viruses, while the nasal-spray flu vaccine contains live, weakened viruses. The nasal spray is normally given to healthy individuals between age 5 and 49. The flu shot can be administered to anyone 6 months or older, regardless of health. Each flu vaccine contains 3 different flu viruses, prevalent that year.

As always, check with your doctor for vaccinations that are best-suited to your personal health profile.