ELI5: Why do we get sick?

Imagine your body is like a strong castle. It has walls, soldiers, and ways to keep bad guys out. Sometimes, tiny little bad guys, called germs (like bacteria and viruses), try to sneak into your castle (your body).

• Germs are everywhere: They're on your hands, in the air, and even on your toys!

Your body has its own army, called the immune system. When germs attack, your immune system fights back! It sends out special soldiers, like white blood cells, to find and destroy the germs.

Sometimes, the germs are too many or too strong, and your body's army can't defeat them quickly enough. This is when you start to feel sick. You might get a fever (your castle heating up to fight the enemy), a cough (your castle trying to kick the germs out), or a runny nose (your castle washing the germs away).

• Being sick is a sign your body is fighting!

Once your body wins, you feel better! And your immune system remembers those germs, so next time they try to attack, your body will be even better prepared to defend itself. That's why you don't always get the same sickness twice!

• Washing your hands is like building a stronger wall around your castle! It keeps many germs from getting inside in the first place.