More than 200 years ago, scientists realized that the immune system could be trained to recognize dangerous microorganisms. This training can happen if the body encounters weakened versions of the microorganisms that are not capable of making the person sick—this is the idea of a vaccine. Vaccines look identical to the real virus or bacteria, but they are changed in the lab to make them weak so they cannot cause disease. The measles vaccine has been changed so that it cannot copy itself properly. When you get the measles vaccine, your immune system sees and investigates this weakened virus and the weak virus in the vaccine does not damage your cells. This vaccination trains the immune system to recognize and "arrest" the real measles virus, if you ever come into contact with it.
In fact, you can think of this immune system training as the police developing a special type of handcuffs that exactly fit the measles viruses.

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