Letter I sent the South China Morning Post; published today, in the Sunday Morning Post:

I used to believe notions that flu could rather magically mutate into a malevolent, super-lethal virus, as suggested by various people, and mentioned by [J.C. Lawrence, in Letters] – and that it would then rampage around the world, killing untold millions.

However, I then saw arguments that this will not happen, based on applying one of the most powerful theories in science – evolution through natural selection – to infectious diseases. In the case of flu, these arguments centre on the need for the disease to be transmitted from one person to another.

Typically, this means transmission from people who are well enough to be mobile – whether walking, riding buses, or taking flights. This in turn means that for a widespread form of human flu to evolve, it must be relatively mild in most people. A disease that greatly sickens and even kills many people will itself quickly die out.

These arguments, from scientists including evolutionary biologist Paul Ewald, explain much that is otherwise puzzling about flu, in both people and birds. The advent of a lethal flu during the First World War is no longer a coincidence, but to Ewald is a direct consequence of the terrible conditions of trench warfare – even very sick people could indeed transmit flu, and the virulent Spanish Flu evolved.

Regular wild bird flus are mild – as birds must fly, even cover vast distances on migration. But cram chickens together in giant poultry farms, and nasty bird flus evolve, including the strains of H5N1 that have caused so much trouble in recent years. The current swine flu is evidently also a product of our farming methods.

Viewing flu as a disease that does not just mutate, but is subject to evolutionary pressures, thus suggests that most of us don’t really have much to fear from flu, including swine flu. There’s no need to panic. But also, care is surely advisable in using drugs like Tamiflu – in case we push flu evolution towards more resistant forms, leading to there being fewer options for treating infections in people whose medical conditions make them especially vulnerable to flu.