Imagine a person with Influenza coughs; a flu virus hitches a ride aboard the droplet that he spewed. The droplet lands on another host and invades the cell. But imagine that cell already harbours another flu virus. What happens next is messy; both the viruses reproduce inside the cell and when the host cell starts manufacturing the viral genes they sometimes get mixed together. The new offspring ends up carrying genetic material from both viruses. This mixing is known as reassortment, a viral version of sex.
Today as the genes of the flu virus are decoded, scientist realise that a quarter of all birds with flu have two or more strains in them at once. Thankfully flu virus has a species barrier, so a flu that affects birds should not affect other animals. But what about animals those are in continuous, close proximity to birds- like humans and hens?
On rare occasions avian influenza virus can pick up human influenza virus genes through reassortment. That is a recipe for disaster because the new strain can easily spread from species to species and humans to humans. Also since it has never circulated among humans before; no one has any defence to it too.
In April 2009, the world became painfully aware that the flu viruses also infect pigs, when an outbreak of swine flu jumped from pigs to humans. This swine flu strain called H1N1, is a sorry tale of industrialised scale pig farming. Pigs have the right biology for reassortment. Some of their receptors can accept both human and bird flu viruses. In 1918 a human flu strain infected pigs (it still makes pigs sick) and in 1970 a bird flu strain in Europe and Asia evolved into a swine flu strain. In 1990 scientist had discovered a "triple reassortment" in pigs having genes of Human-bird and pig flu. The 2009 H1N1 was a cocktail of this triple assortment and the Eurasian bird-to-pig strain and estimated to have evolved on 2008. The virus was unusually swift and had soon spread across many countries from its original place- Mexico.
Similarly the H5N1 that infected hundreds of people in Asia in 2005 was a similar bird- human reassortment. As of now H5N1 can only spread from bird to human and not human to human. But all this is just a matter of time. Our industrialised animal farming is a ticking time bomb, waiting for the perfect cocktail of Influenza virus to one day clean the human population.
For now the least we can do is wash our hands, keep good hygiene. The best we can do is, stop eating meat and stop animal farming all together. Else its tick tick tick…
Trivia: The H in H1N1 stands for 'hemagglutinin' . A spike-shaped protein found on the surface of influenza virus. This protein 'agglutinates' (clumps together) red blood cells, hence the name.