When we get sick, our bodies launch a coordinated defense. 

It's called the immune response. Once a germ's detected, a team of white blood cells, proteins, and chemicals swarm to mount an attack. These cells catch up to the invaders and latch on, destroying them in their wake. 

Or at least that’s what they're supposed to do. 

The body's ability to come back depends on what you have to fight with and how strong you are.

 After we've been sick or hurt, our bodies remember. We learn from the past and develop tools to help us cope. 

Basically, the more we go through something, the better we can handle it if it happens again. Our bodies are prepared to bounce back.

 Or so we think. 

Sometimes an unknown throws the whole thing off.