OK, here we go. A brief technical explanation of the links between human influenza, avial influenza, and pigs:
The species which different types of influenza viruses are able to infect are determined by different forms of sialic acid present on the virus glycoproteins. In particular, this property depends predominately (but not exclusively) on the amino acid at position 226 of the haemagglutinin protein.
This provides a considerable species barrier between birds and humans which is not easily overcome. However, pigs provide a "mixing pot" - able to be infected by both types of virus & thus allowing the passage of avian viruses to humans.
How did avian flu infect humans?
In May 1997, a three year-old boy died of complications of influenza in the intensive care unit of a Hong Kong hospital. This case was the first isolation of an influenza A subtype H5N1 in a human. Subtype H5 influenza viruses can cause lethal avian influenza, a disease that can decimate flocks of domestic poultry.
Fortunately, this outbreak eventually fizzled out, but not before there had been 18 confirmed human cases and 6 deaths. Why did the outbreak go away? The H5 haemagglutinin has an HA226gln sequence - i.e. was an avian virus poorly adapted to humans. There is clear evidence that between its emergence in 1997 and 2004, H5N1 isolates have become significantly more pathogenic for mammals
There is a cyclical pattern to pandemics that appears to be from 59 to 68 years. Take a look at this historical chart:
