11.   According to that theory, the extinction was caused by a disease that jumped between species, as the flu virus does.

12.   But only one had other features that led the researchers to believe that the flu virus was actively replicating when the man died.

13.   But some of these patients actually had bacterial pneumonia, not the flu virus.

14.   By analyzing flu viruses in animals, researchers hoped to trace the source of the genes that mingled with human flu viruses to create such terrifyingly deadly strains.

15.   China, and particularly its poorer provinces, is considered a prime breeding ground for the flu virus which can be transmitted through animal droppings.

16.   Ferrets are particularly suitable test subjects for influenza studies because they contract the same strains of flu virus that bedevil, and sometimes kill, millions of human beings.

17.   Flu viruses usually undergo genetic changes each year, changing so much that they may be considered new strains.

18.   Flu viruses originate in waterfowl and commonly move through several species, chiefly pigs, before infecting humans.

19.   He decided to search for the most likely victims, those who had died quickest, with the flu virus presumably abundant in their lungs.

20.   If they were autopsied, it was long after the flu virus had left their bodies.