Swine Flu is a highly contagious acute respiratory disease.

Influenzas are classified into three major genera: A, B and C. Over thousands of years Influenzas B and C have been domesticated by long circulation in human populations. The Influenza C is a cause of the common cold for instance and B produces a classic winter flu.

Influenza A however remains wild and is very dangerous. It remains primarily among ducks and waterfowl but can cross over to humans and other bird and mammal species. Although the figures are hard to assess domesticated seasonal type A influenza can kill as many as one million people a year. A small increase in the virulence combined with high incidence could cause global problems.

It also has an incredible capacity to evolve rapidly creating modified strains requiring new vaccines – this process is called antigenic drift. However, every human generation or so, a bird or pig version of Influenza A will swap genes with a human type of influenza or acquire mutations allowing it to leap between species – this process is called antigenic shift and signals the imminence of a pandemic. Also, sometimes through a co-infection of a host cell by two different subtypes of influenza can result in a reassortment virus – a hybrid having gene segments from different parents. Influenza A is what Mike Davis calls an “extraordinary shape-shifter”.

Both the 1957 and 1968 flu pandemics are believed to have originated from the mixing of bird and human viruses inside pigs.

The Influenza A subtypes are classified by HxNy. The H stands for hemagglutinin and is the molecular key that influenza uses to ‘unlock and enter’ host cells while the N stands for Neuraminidase which allows the virus ‘escape’ from a dying host. As Davis puts it ‘H is the burglar and N is the escape artist’. There are many subtypes and swine flu is of the H1N1 subtype. But there are other subtypes H1N2, H3N1 or H5N1 commonly known as avian or bird flu.

The N in influenza is more vulnerable than the H. This is what the powerful anti viral drugs Relenza (zanamivir) and Tamiflu (oseltamivir) attack.