It is unusually rather a tough virus. Most viruses disappear, you know, if you don't get them into some other body. But it is a tough virus. It always has surprised me that the eradication campaign worked because it could be grown from various bits of other stuff and, in fact, when they used to dig up old graveyards when they were rebuilding the City of London, they always used to call out some smallpox person or some PH&S person to make sure about this, but I don't think they ever grew any smallpox from any of those sources.

[Q] So, theoretically people were still concerned that there might be, there might be smallpox in, in a body that had been buried several, even 100 years before?

They were concerned, but I think the concern was theoretical because I don't think it has been the event and the reason why smallpox was able to be eradicated was for that very reason. It's not wild in the jungles of Wiltshire or anything. It's not an animal disease, although there are many animal pox viruses which are pretty similar and probably related to the phylogeny of smallpox viruses where they come from. But smallpox spreads from person to person, doesn't hang around, apparently, for a very long time, at least it doesn't seem to be infectious for a long time and the disease, if you're careful, can be identified. So you can... you can, if you're very clever, like Henderson was, and very energetic you can trace a chain of contact which you can control and that isn't the case for many infections. And that, that was the, very theoretically, the basis of eradication, with of course huge political administrative logistic difficulties, but which was eventually achieved, and now is threatened by bioterrorism because there's undoubtedly a lot of smallpox virus in odd corners we don't know about.

[Q] When you talk about odd corners, you mean in laboratories or...

Well, I mean, the former Soviet Union had a huge programme, biological warfare programme, not just smallpox, a lot of other things, which they concealed very, very effectively and with huge mendacity, and then the Union broke up to all these many, I don't remember how many republics. Who's got it where, you know? We don't know about botulinous anthrax. It's almost certain that a number of people in Russia were killed by anthrax by escape from a biological warfare facility and so, you know, it's rather sad, again, like tubercle, after this extraordinary feat of eradicating a major world killer, which it certainly, certainly was, we're not sure if it's really out of danger now.

[Q] Oh right. Right.

I mean there's all the business of weaponising and all that but... because another big lesson about smallpox, which is relevant to many, many other current viruses, is that Jenner published his book in 1798 and it became widespread very quickly – vaccination, and variolation – you know, the one before, was probably also pretty effective in the previous century. So that, from, well, it was eradicated in 1977 so it was about a century and three-quarters from having an effective method to eradication and what's been found with more recent viruses – measles, rubella – is that you have to achieve a very, very high level of control and keep the very high level before you've really any chance of getting rid of the disease, otherwise it springs up again.

[Q] So what would, why do you think that... is was a... that it was possible to do that with smallpox which hasn't, and it hasn't been possible with these other diseases? Is it just, is it about...

Well, I think for that, the thing is. We used to see – you see measles and we saw bi-annually, you couldn't grow virus, virus for love or money any other time. It was there somewhere, sub-clinical infection, chronic infection, chronic carriage, animal – there are various reasons, it's different in different cases, why bugs can't necessarily be found from the patient. It's that really.

[Q] So smallpox must have been always rumbling on in a population somewhere?

That's right. That's right, yes. In density of population is the clue. If it was in a big dense population there'd be recurrent epidemics. If it's drooling around from village to village, you wouldn't know how to deal with it.