An Infection is caused when something unknown enters your body. So, we can think of an infection like an armed bank robber. The bank robber is considered the antigen, which means unknown invader.

To help deal with general infections, the body has defenses that help to keep anything out. These defenses are deemed non-specific, and include things such as mucus, skin, and nose hair. In a bank, the non-specific defense is a metal detector.