What's in a name? A disease by any other name would hurt as badly. But just how confusing a name can be is demonstrated by the outbreak of 'swine flu' that started recently in Mexico. From the name any reasonable person might conclude that this was a disease previously known in pigs that had transferred its attention to human hosts. This reasonable person would be wrong, and there has in fact been no known case of 'swine flu' in a pig. So why did they call it that?

The main strains of the influenza virus are distinguished by two proteins on its surface: the H protein and the N protein. The various versions of these proteins are given numbers, and the swine flu virus is an H1N1. The bird flu virus we were all worried about before the swine flu outbreak is an H5N1, and may still get around to killing quite a few of us. The version of the H1 protein found on a swine flu virus is of the same kind as that found on the porcine influenza virus, which, as its name suggests, infects pigs. However, unlike avian influenza A (H5N1) which is known to have arisen in birds and has been extensively observed killing them in large numbers, it is not yet known exactly where the swine flu virus originated. Other proteins match those found in avian and human influenza.

To conclude: although it is possible that the swine fever virus evolved in pigs, it is not yet known that it did. It is not even known for certain if it infects pigs. It is therefore not known if it is or ever has been a zoonosis. The only known means of transmission at present is from human to human. The high level of concern expressed by the World Health Organisation is due partly to this fact: a novel virus that is only transmitted from animals to humans is an easily contained threat, since contact between humans and animals is relatively easily managed. The worry about H5N1 avian influenza was and is that it might mutate to become infectious between humans. Wherever it originated, this is a step that 'swine flu' has already taken. Until it is known how easily it spreads among humans and how severe a typical infection is there is every reason for public health authorities to keep a very close eye on it and to err on the side of caution in the advice they give, regardless of the low number of deaths so far. The rest of us can carry on cuddling pigs and eating pork as usual.

If you don't believe me, read this.