填空题

Avian flu or H5N1 has infected more than a hundred people so far in at least ten countries. The virus in Asia is killing more than 50% of the humans infected. If this virus learns to (36) ______human to human and (37)______that level of killing humans, we’’d got a global (38)______. Under (39)______ microscope, flu viruses look like spiky creatures. H5N1 may seem unassuming in the lab, but be sure it has ability to cause a public health (40)______. In fact experts say that H5N1 (41)______ strain responsible for the 1918 Flu pandemic. That strain mutated to (42)______ between people and (43)______. killed as many as 15 million around the world. (44)____________. But scientists warn that H5N1 could change to become an explosive killer. (45)____________ Here’’s what I mean. Let’’s say that the chicken farmer has the regular flu which always spreads like a wild fire from person to person. Then he gets infected with H5N1 from one of his chickens. Now both viruses are in the cells where they exchange some of their genes. That’’s called re-assortment. If they shuffle the gene just right, (46)____________. And that is the biggest fear of all.