题目内容：

    Infections that cross over from other species are a deadly problem. The new coronavirus(冠状病毒) is the latest example of a disease that jumped from animals into humans. When infections do this, they can be deadly—and 2019-nCoV is no exception.

Nearly all viruses and bacteria that infect other organisms are completely harmless to people. But a tiny proportion can infect us and cause so-called zoonotic diseases, which come from animals rather than people. Such diseases are a massive problem. They make around 2.5 billion people ill every year and kill 2.7 million. Not all zoonotic diseases cause serious illness, but the Ebola virus, for example, currently kills most of those it infects.

One reason zoonotic viruses can be this deadly is that we lack pre-existing immunity( 免疫) to them. Another is that these viruses aren’t adapted to humans. Viruses that normally circulate among people can develop to become less deadly, as this helps them spread. “They don’t want you to drop dead within a day because you won’t pass it to anyone else,” says Chris Coleman at the University of Nottingham, UK.

To get infected, people need to come into contact with the animal the virus usually infects. This is most likely with domesticated animals. Camels carry the MERS coronavirus that causes sporadic human cases, for instance.

Many viruses that jump into people, like MERS, seldom spread from person to person. They can still infect thousands, though. Rabies is mostly passed on by dog bites, but kills 60,000 people a year. Others, such as Ebola, can spread from person to person, but aren’t very good at it and so cause relatively small outbreaks. The 2019 coronavirus, by contrast, appears quite good at spreading from person to person. We don’t know how deadly it is yet.

Biologists have been warning for decades about the risks of animal viruses spreading to people. There is good reason to worry. The last global pandemic(传染病), the 2009 flu that killed up to 400,000 people, was caused by a strain of flu that came from pigs. And that flu is thought to be a descendant of the 1918 flu, which came  from birds. HIV, which has infected about 75 million people, is now thought of as a human virus. But it jumped from chimpanzees into humans relatively recently, in the 1930s.

Coleman thinks there is little we can do to stop people coming into contact with animals that may carry dangerous viruses. “It’s very difficult to control that,” he says. Instead, he says we need to have vaccines( 疫苗) ready in advance. This could mean creating vaccines that are effective against a wide range of viruses and developing vaccines that require only minor changes to work against a new viral strain, much like annual flu vaccines.

1.What does the underlined word “circulate” in Para.3 probably mean?

A.Pass on. B.Attack each other.

C.Die away. D.Work together.

2.What can we learn from the passage?

A.People must keep away from wild animals.

B.The majority of viruses are a massive problem.

C.The 2009 flu had no connection with the 1918 flu.

D.Vaccines are an effective option to fight against viruses.

3.The passage mainly aims to _______.

A.analyze the origins of viruses

B.explain the consequence of diseases

C.draw people’s attention to zoonotic viruses

D.persuade people to protect the wild animals

4.Which of the following can be the best title for the passage?

A.How Are Vaccines Made?

B.Viruses Infected from Animals

C.A Global Health Emergency

D.Why Isn’t There a Coronavirus Vaccine Yet?