Today we know that influenza is caused by a virus and that it can be spread from person to person in respiratory secretions expelled by coughing, sneezing, and talking.* It is present worldwide even in the Tropics, where it can strike year-round. In the Northern Hemisphere, flu season runs from November to March; and in the Southern Hemisphere, from April to September.
Influenza type A, the most dangerous type of flu virus, is small in size compared with many viruses. It is usually spherical, with projections from its surface. When this virus infects a human cell, it reproduces so rapidly that often within about ten hours, a swarm of between 100,000 and a million new influenza virus “copies” explode from the cell.
The scary characteristic of this simple organism is its ability to change quickly. Because the virus reproduces so rapidly (far faster than the HIV virus), its many “copies” are not exact. Some are different enough to escape the immune system. That is why we face different flu viruses every year, which present a new set of antigens substances that test our immunity. If the antigen changes sufficiently, our immune system has little defense against it and there is risk of a pandemic.
Furthermore, flu viruses also infect animals, and therein lies a problem for humans. The pig, it is believed, can be a host for viruses that infect such birds as chickens and ducks. But it can also be the host for other viruses that infect humans.
Therefore, if a pig becomes infected by both types of viruses—one sort that infects animals and another sort common to humans—the genes of the two strains can get mixed together. The result can be a totally new strain of influenza, one to which humans have no immunity. Some feel that farming communities where poultry, swine, and people live in close proximity—as is often the case in Asia, for example—are likely sources of new flu strains.