Your body has lots of ways to keep you from getting sick or to help you get better more quickly when you do get sick. Your first line of defense is your skin and the membranes inside your body. These keep nasty bacteria and viruses away just like a wall. When you get a cut, why do you have to be careful to keep it clean? So that you do not get an infection.
Sometimes though, your skin is not enough, and you do get sick. When you get a fever, that is a sign that your body is trying to fight whatever is causing you to be sick. There are some clever ways your body can fight infection. After your body fights off an infection by a virus for the first time, it can form a memory of what that virus looks like. That way, you would not get sick from that virus again. You will recognize the "bad guy" and fight back. We call this memory "adaptive immunity" and it is why people usually only get the chicken pox once, and it is also why vaccines work.

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Go to source: https://kids.frontiersin.org/article/10.3389/frym.2019.00102