听力原文： Today, as AIDS licks the whole world, we should no longer term it as taboo, but take a scientific view of it. AIDS is the fifth leading cause of death among persons between ages 25 and 44 in the United States. About 47 million people worldwide have been infected with HI~ since the start of the epidemic. AIDS stands for 'Acquired Immune Deficiency Syndrome'. It is caused by the Human Immunodeficiency Virus(HIV). AIDS is the final and most serious stage of HIV disease, in which the signs and symptoms of severe immune deficiency have developed. HIV has been found in saliva, tears, nervous system tissue, blood, semen, vaginal fluid, and breast milk. However, only blood, semen, vaginal secretions, and breast milk have been proven to transmit infection to others. Transmission of the virus occurs through sexual contact, blood transfusions or needle sharing, and from mother to child. A pregnant woman can passively transmit the virus to her fetus, or a nursing mother can transmit it to her baby. Other transmission methods are rare and include accidental needle injury, artificial insemination through donated semen, and through a donated organ. HIV infection is not spread by casual contact such as hugging and touching, by touching dishes, doorknobs, or by mosquitoes. It is not transmitted to a person who donates Blood or organs. However, it can be transmitted to the person receiving blood or organs from an infected donor. This is why blood banks and organ donor programs screen donors' blood and tissues thoroughly. How many people worldwide have been infected with HIV since the start of the epidemic?