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.

　　That's exactly what is happening in small neighborhoods around the country from Orange County, California, where 22 measles cases were reported this month, to Brooklyn, N.Y., where a 17-year-old caused an outbreak last year.

　　The resistance to vaccine has continued for decades, and it is driven by a real but very small risk. Those who refuse to take that risk selfishly make others suffer.

　　Making things worse are state laws that make it too easy to opt out(决定不参加)of what are supposed to be required vaccines for all children entering kindergarten. Seventeen states allow parents to get an exemption(豁免), sometimes just by signing a paper saying they personally object to a vaccine.

　　Now, several states are moving to tighten laws by adding new regulations for opting out. But no one does enough to limit exemptions.

　　Parents ought to be able to opt out only for limited medical or religious reasons. But personal opinions?Not good enough. Everyone enjoys the life-saving benefits vaccines provide, but they'll exist only as long as everyone shares in the risks.

1.The first two paragraphs suggest that　　　.

　A.a small number of measles cases can start a dangerous trend

　B.the outbreak of measles attracts the public attention

　C.anti-vaccine movement has its medical reasons

　D.information about measles spreads quickly

2.Herd immunity works well when　　　.

　A.exemptions are allowed

　B.several vaccines are used together

　C.the whole neighborhood is involved in

　D.new regulations are added to the state laws

3.What is the main reason for the comeback of measles?