31.   The CDC influenza surveillance system aims primarily at identifying the influenza virus, not the many other ones that are now causing upper respiratory tract infections.

32.   The influenza virus does travel easily through the air as a passenger on water droplets, so a mask may head off its transmission.

33.   The influenza virus is enclosed in a membrane, and in order to infect a cell its membrane and that of the target cell must fuse.

34.   The flu is an infection of the respiratory tract that is caused by the influenza virus.

35.   The Hong Kong cases are the first in which an avian influenza virus apparently has jumped to humans without first going through pigs or other animals.

36.   The optimal temperature for avian influenza viruses tends to be higher than that for human viruses.

37.   The research here is part of a continuing scientific battle of wits to understand why new human influenza viruses pop up when they do.

38.   The Sydney strain is a Type A influenza virus, one of the two major branches of the influenza family.

39.   The success of the gonorrhea experiment in mice bodes well for other nasal vaccinations in animal trials, including meningitis and the influenza virus.

40.   The vaccine is produced after health researchers decide which of the influenza viruses are most likely to cause the flu that year.