Influenza A virus is distinctive in its genetic instability, which probably makes permanent immunity to the disease impossible, no matter how many times it is contracted. This, plus its short incubation period of 1 to 2 days and the ease with which it passes from person to person, enables the disease to swing around the globe in pandemics and, between them, to maintain itself not only in epidemics among the previously unexposed and newborn, but also among the previously exposed and adult majority. Between pandemics the proteins on the shell of the virus undergo slow but constant genetic change, rendering acquired immunity ever more obsolete. This genetic instability is the likeliest explanation for the fact that even during pandemics the virus seems to change sufficiently to produce repeating waves of the infection, often two and three in a given locale. Several times a century, the virus has changed radically, rendering obsolete the immunologic defenses of the great majority of humans vis-à-vis influenza, including immunologically experienced adults. In the mildest of these pandemics, millions fall ill and thousands die.

The cause of the major changes in the virus that set off the pandemics is still a matter of mystery. Many theories have been devised, including those pertaining to the influence of sun spots and such. Currently the three most plausible theories are the following:

1. The influenza A virus currently circulating through the human population undergoes a series of mutations, which rapidly and radically transform the virus into an infection-producing organism for which human immune systems are unprepared.

2. An animal influenza virus abruptly gains the ability to cause disease in humans, with the same results.

3. A human influenza virus and an animal influenza virus recombine ("cross-breed") to produce a new virus that retains its capacity to infect humans but has a surface with which human immune systems are unfamiliar.

In the present state of research, the first of these three seems the least likely and the last the most likely explanation. Nothing, however, is certain yet, and the cause of influenza pandemics remains unknown.

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