Until much more is known, "influenza" includes at least three types of infection, all of which present the same symptoms. The first of these is the catch-all, "acute febrile catarrh," from which the term "cat fever" is derived. This includes the influenza-like diseases from which specific viruses have not yet been isolated. The second form is influenza A, traceable to the specific virus isolated in England in 1933 or to related strains. It is this virus which seems to be largely responsible for the present epidemics and which is so widespread that it has been identified in England, in Martinique, in Minnesota, and in army cases in Michigan. The third form is influenza B, also due to a specific virus; this virus was isolated in New York State in 1940 and then traced back to a California epidemic in 1936.
Both of these viruses, A and B, are of world-wide distribution, but are apparently unrelated; both can cause epidemics, and alone or together they might cause a pandemic. Except for the fact that influenza A is often more severe than influenza B, their differentiation is important to the victims of the disease mainly from the point of view of immunity, since one apparently offers no protection against the other.
Epidemic influenza is characterized by its sudden appearance and rise to a peak in three or four weeks, followed by a prompt subsidence from six to eight weeks after the onset. Although from 10 to 40 per cent of a population may be attacked, the mortality is low. Important in its spread are not only the frank cases of obstinate people who refuse to go to bed, but also persons who are in the incubation period and who innocently remain at large, and finally the cases of "subclinical" infection—that is, of victims who have the disease like "walking typhoid." Such persons can spread the infection and may include as high as one fourth of the population. Convalescent carriers may also be important, but the concert coughers, however high their nuisance value, may be relatively innocuous.
Now what steps can be taken to prevent influenza? Avoidance of exposure is preached but too seldom practiced. Crowded trains, streetcars, and buses—here is the environment in which respiratory infection flourishes. While thousands of civilian workers and members of the armed forces are forced to take this risk, anyone who feels a cold coming on and has an ivory tower available should stay in it.
Two types of vaccination against influenza are being studied. In one of them the vaccine is put under the skin. In the other it is applied to the mucosa of the respiratory tract to increase resistance at the local barrier which the virus must pass to cause infection. Extensive trial of this type of vaccination has been made among Australian troops and in Russia. In general it may be said that vaccination against influenza is still in the experimental stage and that, while it may not prevent infection, it may modify the course of the disease to a milder form with fewer complications.