Chicken pox is a contagious disease caused by a virus that gives you an itchy rash covered in small, fluid-filled blisters all over your body. Before the chicken pox vaccine became common in the 1990s, almost every child caught it at some point, usually between ages 3 and 10.

The illness starts with fever and tiredness, but then comes the trademark rash: hundreds of tiny red bumps that turn into blisters, often covering your chest, back, face, and even inside your mouth. The worst part? They itch like crazy, and scratching them can leave scars. Kids with chicken pox usually stay home from school for about a week while the blisters scab over and stop being contagious.

Most children who get chicken pox recover completely, though they feel miserable while they have it. The vaccine has made chicken pox much less common today, protecting kids from those itchy, uncomfortable days that previous generations simply had to endure.