31.   The new compound is a neuramindase inhibitor, meaning that it blocks an enzyme the flu virus needs to infect cells.

32.   The scientists say they have stripped the human immunodeficiency virus of its ability to cause disease, while leaving intact its ability to infect human cells.

33.   The virus can be injected into the fluid-filled compartment behind the retina, where it will infect retinal cells.

34.   Two years ago a British virologist, Robin A. Weiss, proved that PERV could infect human cells in the test tube.

35.   Unlike bacteria, which are content simply to suck up nutrients from a culture dish, viruses must infect a cell before they can reproduce.

36.   When a cell is infected with a virus, like the AIDS virus, it puts fragments of viral proteins on its surface, like flags signaling distress.

37.   When researchers found that the virus could infect human cells, clinical trials exposing people to pig cells were briefly stopped.

38.   When the virus infects a cell, it must first induce the cell to divide in order to replicate and then go on to infect other cells.

39.   With their ability to infect cells, viruses are useful in gene therapy because doctors can use them to deliver healthy DNA to patients.

40.   Or is it replicating at a very low level and infecting new cells to keep the reservoir going as old cells die?