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．

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．

63．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

64．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

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

A．The overuse of vaccine．

B．The lack of medical care．

C．The features of measles itself．

D．The vaccine opt-outs of some people．