填空题
The year was 1959. Location: the central African city of Leopoldville, now called Kinshasa, shortly before the waves of violent rebellion that followed the liberation of the Belgian Congo. A seemingly healthy man walked into a hospital clinic to give blood for a West-ernbacked study of blood diseases. He walked away and was never heard from again. Doctors analyzed his sample, froze it in a test tube and forgot about it. A quarter century later, in the mid-1980s, researchers studying the growing AIDS epidemic took a second look at the blood and discovered that it contained HIV, the virus that causes AIDS.
And not just any HIV. The Leopoldville sample is the oldest 41 of the AIDS virus ever isolated and may now help solve the 42 of how and when the virus made the leap from animals (moneys or chimpanzees) to 43 , according to a report published last week in Nature . Dr. David Ho, 44 of the Aaron Diamond AIDS Research Center in New York City and 45 of the study"s samples, says DNA pushes the putative origin of the AIDS 46 back at least a decade, to the early "50s over even the "40s.
Over the 47 15 years, scientists have identified at least 10 subtypes of the 48 virus. But they couldn"t tell whether they were seeing 49 on one changeable virus or the handiwork of several 50 viruses that had made the jump from primates to man. A 51 look at the genetic mutations in the Leopoldville sample strongly 52 that all it took to launch the AIDS epidemic was one unlucky 53 of events.
By comparing the DNA of the 1959 virus with that of 54 taken from the "80s and "90s, Ho and his 55 constructed a viral family tree in which the Leopoldville isolate sits right 56 the juncture where three subtypes branch out. The 39-year-old 57 is also strikingly similar to the other seven subtypes. The clear implication: all the viral 58 can be traced back to a single event or a closely related group of 59 . One theory is that AIDS started through contact with infected monkeys in a 60 area and spread to the rest of the population through urbanization and mass inoculations.
The findings underscore how rapidly HIV can adapt to its surroundings, making it devilishly difficult to develop effective vaccines. No one knows how many more subtypes of HIV will sprout in the next 40 years, but chances are they will be every bit as lethal as the ones we see today, if not more so.