We have studied the AIDS viruses for simian and human viruses and there's one particular result which showed that the sequences we looked at seemed to have two regions of mutation. About 20% of the positions are almost constant. In all viruses we find they are the same, exactly agree completely, and moreover they agree with positions in other retroviruses which have nothing to do with AIDS... there are many retroviruses known nowadays. So that tells us that the evolution of the virus is that of an ordinary virus, it has a long history, coming along with the evolution of the species... in other words they belong to one big family of retroviruses. But that's only 20%. Then we find there are 70% of the positions which have a pretty long substitution time, on average a thousand years. In other words it takes a thousand years to substitute every of these positions, which still means after a thousand years you have a completely different virus because if you substitute all the positions, of course, you can only substitute by those which survive, which are fit enough.

[Q] Ten are left.

Yes. Now, yes, twenty and seventy is ninety, so 10%. And they are hypermutable.

[Q] 10%.

They change with the average time of thirty years, that's almost the incubation time of those diseases. And they are the troublemakers, because they cause the virus to escape the immune response. So the evolution of the virus is pretty well known, we find these... evolution in the 70% among simian HIV-1 and HIV-2. But within the HIV-1 group we have predominantly these recent mutations with a half-time of thirty years and, as I said, with the other viruses there are still 20% agreement. So we know pretty well about evolution, we have done that same for other viruses. Another one which evolves quite rapidly is influenza virus.

[Q] Just to go back, the result out of these calculations means the HIV is approximately how old?

The HIV in the form we find it nowadays has come about within the last thousand years, not longer than that. Before that it belonged to the family of retrovirus which have no symptoms like AIDS or don't cause symptoms like AIDS, which would mean also, it's an important finding. It would mean those viruses come and go, there might be other plagues coming up in the future... of viruses of which we don't know anything nowadays yet, and it might be that the AIDS virus after some time gets harmless again.