英语六级阅读备考：快速阅读练习(7)_大学英语阅读

英语六级阅读备考：快速阅读练习(7)

　　HIV kills by slowly destroying the immune system. Several weeks after initial infection, flu-like symptoms are experienced. Then the immune system kicks-in, and the virus mostly retreats into hiding within lymph tissues. The untreated, infected individual usually remains healthy for 5 to 15 years, but the virus continues to replicate in the background, slowly obliterating the immune system.

　　Eventually the body is unable to defend itself and succumbs to overwhelming opportunistic infections that rarely affect healthy people. Acquired Immune Deficiency Syndrome is the name given to this final stage of HIV infection, and is characterized by multiple, life-threatening illnesses such is weight loss, chronic diarrhea, rare cancers, pneumonia, fungal conditions and infections of the brain and eye. Tuberculosis has become especially prevalent in AIDS victims.

　　Natural Born Killer

　　Genetic analyses hint that ancestral primate HIV may have been born a million years ago when a chimpanzee virus hybridized with a related monkey variety. However researchers believe it was not until the 1930s that this jumped to humans eating chimp meat in Central Africa. That variety became HIV-1the most widespread type. A second type, HIV-2, restricted to West Africa, was probably contracted in the 1960s from monkey meat.

　　Another theory was that the AIDS pandemic was accidentally started by doctors testing a polio vaccine in the 1950sdetailed in Edward Hoopers book The Riverbut this has been severely criticized by other researchers.