英语六级阅读备考：快速阅读练习(7)

　　Passage 7

　　HIV AIDS

　　AIDS has now surpassed the Black Death on its course to become the worst pandemic in human history. At the end of 2004. 20 million people had been killed by it, and twice that number are currently infected with HIV. Barring a medical breakthrough, it could claim the lives of some 60 million people by 2015. AIDS exerts a terrible toll on societies, crippling their economies, decimating their labor forces and orphaning their children.

　　Nine out of 10 people living with HIV are in the developing world; 60 to 70% of those are in Sub-Saharan Africa. But the disease is spreading in every region, with fierce epidemics threatening to tear through countries such as India, Russia and the islands of the Caribbean. The statistics are sobering in some Southern African towns 44% of pregnant women are HIV positive, in Botswana 37% of people carry the virus.

　　Immune Assassin

　　The human immunodeficiency virus is a retrovirusa virus built of RNA instead of more typical DNA. It attacks the very cells of the immune system that should be protecting the body against itT lymphocytes and other white blood cells with CD4 receptors on their surfaces. The virus uses the CD4 receptor to bind with and thereby enter the lymphocyte. HIV then integrates itself into the cell sown DNA, turning the cell into a virus-generating factory. The new viruses break free, destroying the cell, then move on to attack other lymphocytes.

　　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.

　　AIDS must have been circulating in the US and Africa during the 1970s. But it was not recognized until 1981 when young gay men and injecting drug users, in New York and California, started to be diagnosed with both an unusual skin cancer called Kaposi s sarcoma, and lethal pneumonias. By the end of that year 121 people in the US had diedthat number would rise to 17,000 over the next six years.

　　Government scientists predicted that the mysterious immune-debilitating illness was due to an infectious agent. In 1984 that agent was identified as HIV by Luc Montagnier of the Pasteur Institute in is, France, and Robert Gallo of the National Cancer Institute in Washington DC, US.

　　Soon after the appearance of AIDS in the US, the disease was detected in Europe too and epidemics affecting heterosexual men and women sprang up at an alarming rate in Sub-Saharan Africa. Today one in five people in that region are living with the virus. AIDS epidemics also threaten to devastate the worlds most populous nations, if action is not taken to bring them under control.

　　Defensive Measures

　　HIV is found in body fluids such as: blood, semen, vaginal fluids and breast milk. It can be passed on through penetrative sex, oral sex and sharing contaminated needles when injecting street drugs or in hospitals. It can also be transmitted from a mother to her baby during pregnancy, childbirth or breastfeedingthough many children escape infection. HIV cannot be passed on through kissing, coughing, mosquito bites or touching.

　　Health authorities are focusing on prevention as a key method to limit the spread of the epidemic. Educational programs preach abstinence from sex, monogamy and safer sex using condoms, as ways to protect against infection. Many countries give away free condoms and offer needle exchange programs to try and limit transmission among injecting drug users. Microbicides in the form of creams that prevent transmission of HIV may soon offer another method of protection.

　　A vaccine, as an alternative method to prevent HIV infection, may still be many years away. This is partly because the virus mutates so rapidly. A vaccine may not only have to prime antibodies to attack the virus but might also need to increase T-cell production. Vaccine trial; have been undertaken in South Africa, Kenya, the US and Thailandthough most have yet to yield promising results. Controversial vaccines made from the blood of HIV carriers, have been tested is Nigeria and Thailand.

　　Anti-retroviral Cocktails

　　There is no cure for AIDS, but a range of drugssome of which have unpleasant side-effectsare available to slow its progress. Other drugs are used to treat opportunistic infections or AIDS symptoms Even some herbal treatments have been investigated.

　　Most anti-HIV drugs aim at stalling viral replication. Nucleoside analogues such as AZT and also non-nucleoside reverse transcriptase inhibitors , attack the action o! the viral enzyme reverse transcriptase. This prevents it from creating functional DNA which would otherwise integrate into the DNA of infected cells.

　　A third class block protease, an enzyme essential for generating functional virus particles. Protease inhibitors are the most effective of the three types of drugs, and AIDS mortality fell dramatically in the US when they were first licensed during the late 1990s. Fusion inhibitors are a newer type of drug that work by stopping HIV from binding with CD4 receptors that it uses to enter cells. Drugs that block another enzyme, integrase , are also under development.

　　AIDS drugs are often administered in combination cocktails of three or more kinds simultaneously, as this helps slow the rate at which HIV develops resistance to drugs. But the virus is able to evolve rapidly and can eventually outpace the drugs if treatment regimens are not followed rigorously.

　　Though drugs are widely available in Western countries, their expense means they are unavailable to the vast majority of AIDS sufferers. International bodies are working towards widening access to treatment in the developing world. Some companies in countries such as India and Thailand are now producing cheap generic copies of drugs.

　　Staggering Toll

　　The economic and social burden of AIDS exerts a great toll on developing nations in addition to that exerted by mortality itself. AIDS is hindering development and leading to negative population growth in some of the most seriously affected nations, such as Botswana.

　　This excessive AIDS mortality is causing a great demographic shift, wiping out young adults in the prime of their lives. This leaves children orphaned, and is destroying workforces and economies. Some predict that 50 million children in Sub-Saharan Africa will have been orphaned by 2010. The labor forces of 38 AIDS ravaged countries will be up to 35% smaller by 2020, because of AIDS.

　　The effect of AIDS on agricultural communities in Southern Africa is even leading to food shortages.

　　Social stigma and discrimination is yet another problem for many AIDS sufferers, especially in Asian nations