The influenza viruses that circulate during an ordinary flu season are bad enough. But every so often, a new and more dangerous strain of influenza virus starts circulating. The worst strains of flu are hybrids of ordinary flu viruses with strains that don’t normally infect human beings. Some researchers believe that the dangerous new strains of flu arise when unrelated strains of flu virus meet and swap genes in domestic pigs.
There are two basic ways in which a new strain of the flu can get started. One is through ordinary mutation. Influenza viruses mutate incredibly fast because their genes are in a single strand of RNA. Without the “backup copy” that you find in double-stranded DNA, they can’t use their host cell’s proofreading equipment to find and correct copying mistakes in their genes. Here’s a technical explanation, for people who like math. It explains why influenza viruses mutate about 300 times faster than poliovirus and even about 10 times as fast as HIV.
Normally, an unstable genome would be a bad thing. That’s because most mutations are harmful to the virus itself. A virus with an unstable genome would tend to die out unless it reproduced itself in large numbers very quickly and had the ability to recombine with related viruses. But under those circumstances, the genetic instability turns into an advantage! It enables the virus to stay one step ahead of the host’s immune system. We can get the flu again and again because the flu that’s going around this year is a little bit different from the strain that went around last year. The antibodies we made last year aren’t a perfect match for this year’s flu.
The very worst flu strains are the ones that are radically different from the ones that had been going around in earlier years. They can infect more people and cause a more serious illness. These pandemic flu strains seem to represent a combination of genes from different strains of flu: some ordinary human flu strains plus some unfamiliar type of flu that normally infects wild birds. That’s why there’s been so much hysteria lately about “avian flu.” Pigs seem to be a perfect “mixing vessel” for new and deadly strains of flu, such as the “swine flu” that caused millions of deaths at the end of World War I.
Preventing future flu pandemics would be simple. All we have to do is stop raising domestic poultry and domestic swine. At the very least, the use of antiviral drugs in agriculture should be banned. Otherwise, they will no longer be useful against flu.