Measles(麻疹), which once killed 450 children each year and disabled even more, was nearly wiped out in the United States 14 years ago by the universal use of the MMR vaccine(疫苗). But the disease is making a comeback, caused by a growing anti-vaccine movement and misinformation that is spreading quickly. Already this year, 115 measles cases have been reported in the USA, compared with 189 for all of last year.
The numbers might sound small, but they are the leading edge of a dangerous trend. When vaccination rates are very high, as they still are in the nation as a whole, everyone is protected. This is called “herd immunity”， which protects the people who get hurt easily, including those who can’t be vaccinated for medical reasons, babies too young to get vaccinated and people on whom the vaccine doesn’t work.
But herd immunity works only when nearly the whole herd joins in. When some refuse vaccination and seek a free ride, immunity breaks down and everyone is in even bigger danger.