41.   But concern rose when researchers recently discovered the genes of all pigs harbor a previously unknown virus that test-tube experiments showed could infect human cells.

42.   Normally, when a virus infects a cell, the cell displays bits of viral proteins on its surface like red flags to alert the immune system.

43.   Research has shown that some of them can infect animal cells in the laboratory.

44.   Some studies in which humans were exposed to pig cells have suggested that PERVs do not infect human cells.

45.   Some virus strains need to latch onto those structures to infect cells, a necessary step before they can reproduce.

46.   The idea is to let the virus infect the cells in the airways so that the cells carry in the needed gene.

47.   These particles are released from the cells and can then infect other cells, spreading the infection throughout the body.

48.   They found that the typhoid fever germ uses the protein made by normal CFTR to infect gastrointestinal cells.

49.   When they infect the skin cells, the cells begin pumping out the proteins, and the body mounts an immune response.

50.   Without rev, HIV can infect cells but not subvert and destroy them.