Measles is on the rise in the United States. So far this year, the number of cases is about 17 times what it was, on average, during the same period in each of the four years before, according to the Centers for Disease Control and Prevention. Half of the people infected — mainly children — have been hospitalized. It’s going to get worse, largely because a growing number of parents are deciding not to get their children vaccinated against measles as well as diseases like polio and pertussis. Unvaccinated people, or those whose immunization status is unknown, account for 80% of the measles case…