A bite from a tsetse fly is an extremely unpleasant experience. To make matters worse, several species of tsetse fly can transmit diseases. One of the most dangerous is a parasite(寄生虫) that causes "sleeping sickness".

After the initial bite, sleeping sickness symptoms often start with a fever, headaches and aching muscles. As the illness goes on, those infected become increasingly tired, which is where it gets its name.

It is worth noting that sleeping sickness is no longer as deadly as it once was. In the early 20^th Century several hundred thousand people were infected each year. By the 1960s the disease was considered "under control" and had reached very low numbers, making its spread more difficult. But in the 1970s there was another major infection, which took 20 years to control. Since then, better screening programs and earlier treatments have reduced the number of cases dramatically. In 2000 this figure dropped to fewer than 3,000. The World Health Organization (WHO) hopes the disease will be completely removed by 2020. More problematically, a series of new studies have shown that the parasite is more complicated than previously believed.

Sleeping sickness has always been considered—and analyzed—as a blood disease, because the parasites can readily be discovered in the blood of its victims. However, in a recent study found that the parasite can stay in the skin and fat, as well as in the blood. There may even be a higher density（密度）of the parasite in the skin than in the blood. That means a person can have no symptoms but still both harbor the disease and spread it. The finding could explain the mysterious 1970s infection, and why the disease can spring up in areas that had previously been cleared.

Dolphins are social and intelligent animals. And, like the way humans maintain relationships by hugging or giving a handshake, dolphins breathe together at the same time when they come up from the water for air. This shared act is important for creating social connections. But sharing the same air and area is also spreading an infectious and deadly disease among the dolphins.

Janet Mann and other scientists are trying to understand the virus. They discovered it in the water off the American states of Virginia and Maryland. It is now spreading to other Atlantic coast dolphins.

Mann explains that the dolphin virus is in the lungs. “When dolphins breathe together at the surface, they’re sharing tiny droplets just like we do when we’re talking with each other,” she said.

During her 35 years of studying dolphins, Janet Mann has noticed that even though dolphins have close friends, they visit other dolphins and leave the groups often. Following the social lives of dolphins in the Chesapeake Bay has permitted researchers to identify over 2,000 dolphin individuals. They can remember them by their special shapes and markings on their back fins.

Two researchers, Melissa Collier and Ann-Marie Jacoby, saw two dolphins. A third dolphin joined his friends. All three dolphins came to the surface of the water and breathed together. “This is typical, male behavior. The males stay pretty coordinated (协调) with each other. The females sync (同步), but not as regularly. They syne mostly with their offspring (幼崽).” Mann says. This behavior pattern might explain why more male dolphins have died in the most recent outbreak of the virus.

Viruses are naturally occurring in the wild, but human activities in the ocean can make the virus worse by wakening environments and populations even more. Pollution from carbon and plastics, limited food sources, along with ocean warming from climate change, harm the animals. These factors weaken the dolphins’ immune systems. “So, they are extremely vulnerable (脆弱的,易受伤的) to virus outbreaks.” Mann says.

Winter is often quite cold, which often makes us stay indoors more instead of heading out to exercise. Worse still, there are many viruses(病毒) doing their best to make you sick. And sneezing, coughing and nose blowing make the rooms a habitat for viruses. 【小题1】Touching a doorknob or shaking a hand may make you get sick. Although it may sound terrible, there are still some things you can do to avoid viruses. Here are several simple tips to stay healthy in winters.

Wash your hands. You pick up viruses everywhere and they live on your hands, so wash your hands and do it often. 【小题2】. But if that’s not available, use an alcohol-based hand sanitizer (消毒杀菌剂).

【小题3】:Avoid touching your eyes, nose and mouth with your hands. That’s one way viruses can get into your body.

Get the flu shot(疫苗). 【小题4】, but getting the flu shot lowers your chances of getting the flu. For example, getting the flu shot last year resulted in a 34% lower risk of coming down with the flu.

Avoid sick people. 【小题5】 Viruses spread very easily through the air, so keep at least six feet between you and the sneezing, sniffling and coughing person.