The immune system has cells, sometimes called memory cells, which remember diseases. If these cells meet a disease, they keep track of what it looks like so they can recognize it later. When the memory cells meet up with the disease again, they recognize it and know they need to get rid of it. They call in the other parts of the immune system to get rid of the disease. In some cases, memory cells can recognize a disease without ever having to meet up with it, which is called “natural” immunity. In other cases, the cells need some help to become familiar with a disease.

That help comes in the form of a vaccine. The vaccine takes a form of the disease that doesn’t make you sick and introduces it to the memory cells so they know what to look for later. If the memory cells ever bump into the disease again, they know to call in other cells in the immune system to protect the body and get rid of the disease. The memory cells of a child keep track of disease well into adulthood, preventing such disease by getting rid of them quickly. In this way, vaccines help the immune system by making it easier to remember diseases.

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