Chickenpox is a contagious disease that causes an itchy rash of small red bumps and blisters all over the body, along with fever and tiredness. The disease spreads easily when an infected person coughs or sneezes, or when someone touches the fluid from the blisters.

Before a vaccine was developed in 1995, nearly every child caught chickenpox at some point, usually staying home from school for about a week while the blisters scabbed over and healed. The worst part was the intense itching: kids had to resist scratching the bumps, which was incredibly difficult but important because scratching could leave permanent scars or cause infections.

The disease is caused by a virus called varicella-zoster, which is also responsible for shingles, a painful condition that can affect people decades after they've had chickenpox. Thanks to the vaccine, chickenpox is now much less common than it used to be.