Text 4

The World Health Organization says as many as 10 million persons worldwide may have the virus(病毒)that causes . Experts believe about 350 thousand persons have the disease. And one million more may get it in the next five years. In the United States, about 50000 persons have died with . The country’s top medical official says more than 90 percent of all Americans who had the virus five years ago are dead. There is no cure for and no vaccine (疫苗) medicine to pr it. However, researchers know much more about than they did just a few years ago. We now know that is caused by a virus. The virus invades healthy cells including white blood cells that are part of our defense system against disease. It takes control of the healthy cell’s genetic(遗传的)material and forces the cell to make a copy of the virus. The cell then dies. And the viral particles move on to invade and kill more healthy cells. The virus is carried in a person’s body fluids. The virus can be passed ually or by sharing instruments used to take intravenous (静脉内的) drugs. It also can be passed in blood products or from a pregnant woman with to her developing baby. Many stories about the spread of are false. You cannot get by working or attending school with someone who has the disease. You cannot get it by touching, drinking glasses or other objects used by such persons. Experts say no one has gotten by living with, caring for or touching an patient. There are several warning signs of an infection. They include always feeling tired, unexplained weight loss and uncontrolled expulsion of body wastes(大小便失禁). Other warnings are the appearance of white areas on the mouth, dark red areas of skin that do not disappear and a higher than normal body temperature.

The fifth paragraph is mainly about ______.