21.   No one knows whether these viruses could infect a pig organ recipient or whether that would be dangerous.

22.   Recently, for example, a group of scientists in Montana discovered that when the virus first infects a cell, it undergoes a slight change in surface configuration.

23.   Scientists think this often occurs when a flu virus from a bird and one from a human simultaneously infect a pig.

24.   Similarly, in the laboratory, the virus cannot infect their cells.

25.   Someone drops a vial and the virus infects the whole staff, who must find a cure as strength drains from their bodies.

26.   The AIDS virus has infected more people in South Africa than anywhere else in the world.

27.   The AIDS virus mainly infects certain white blood cells known as T cells.

28.   The scientists know, from looking at genetic variants of human AIDS, that the virus infected humans on at least several occasions before becoming a full-fledged epidemic.

29.   The second arm of the immune system, cell-mediated immunity, includes several types of white blood cells that destroy cells that have been infected by viruses.

30.   The T-cells seek out and destroy cells that the viruses have infected.