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Infections are the most common cause of human disease.  Disease-causing microbes (pathogens) attempting to get into the body must move past the body’s external armor, usually the skin or cells lining the body’s internal passageways, and your immune system if these microbes get inside.  Your immune system works because is able to tell if an invader (virus, bacteria, parasite, or other another person's tissues) has entered it—even if you aren't consciously aware that anything has happened. Your body recognizes this invader and uses a number of different tactics to destroy it.

Vaccines help the body’s immune system prepare for future attacks. Vaccines consist of killed or modified microbes, parts of microbes, or microbial DNA that trick the body into thinking an infection has occurred.  A vaccinated person’s immune system attacks the harmless vaccine and prepares for invasions against the kind of microbe the vaccine contained. In this way, the person becomes immunized against the microbe: if re-exposure to the infectious microbe occurs, the immune system will quickly recognize how to stop the infection.

The pages within this section explain more in detail about how your immune system works to prevent you from getting sick. Knowing how your immune system works may help you understand how vaccines work with your immune system to protect you.