Measles is a virus that can make people very sick. In the past, scientists developed a way to make a vaccine which can keep people from getting sick with measles. The vaccine is usually given to children in two doses. In the United States, the vaccine was very successful and the number of people getting measles was very low.
Recently, the number of people getting measles has gone up again. This is called a resurgence. This is happening because some parents have decided not to give their children the vaccine. This has caused more people to get sick with measles, and it has made it harder to protect everyone from the virus.