10 outbreaks of a poxvirus disease had occurred in laboratory colonies of primates in Europe and North America between 1958 and 1966, which investigations showed to be caused by an Orthopoxvirus and the disease had been called 'monkeypox'. To exclude the possibility that it might represent an animal reservoir of smallpox virus, WHO organized a meeting of poxvirus experts from Australia, Japan, the Soviet Union, the Netherlands and the United States, called an 'Informal Group on Monkeypox and Related Viruses', which met first in Moscow in April 1969 and biennially thereafter. The first meeting, for which I acted as rapporteur, agreed that monkeypox virus could be readily distinguished from all other orthopoxviruses by its biological properties. A year later, sporadic cases of a human disease clinically very similar to smallpox were discovered in Côte d'Ivoire, Liberia, Nigeria and Zaire, all in areas where there had not been a case of smallpox for over a year. All of these were shown to be caused by monkeypox virus.

I had missed the second and third meetings of the Informal Group on Monkeypox and Related Infections in 1971 and 1973, but was asked to be chairman of the meeting in Geneva in 1976. The group had been expanded from nine to 17 experts in poxvirus virology and epidemiology. During the interval, there had been 21 cases of human monkeypox in six countries in West and central Africa and these had all been carefully investigated. Two were possibly transmitted from one person to another. Blood had been collected from monkeys and other wild animals, notably squirrels and rodents in the vicinity, for serological tests and attempts to isolate virus; antibodies had been detected in several species of monkeys and in rodents.

A more puzzling observation was that four isolates of a poxvirus that was indistinguishable from variola virus (called 'whitepox' virus) had been made from the kidneys of a chimpanzee, a monkey and two rodents in an area of Zaire near where seven cases of human monkeypox had occurred. The isolates had been made by virologists, headed by Dr S. Marennikova, in the Moscow Research Institute for Viral Preparations, which was one of the two WHO Collaborating Centres on Smallpox and Other Related Infections which had been established by Henderson in 1967.

The main objective of this meeting, of which I was again Chairman, was to discuss the findings of Dr Marennikova's group that viruses having some of the characteristics of whitepox virus arose as variants of monkeypox virus. Comparisons were drawn between the known white pock mutants of both cowpox and rabbitpox viruses, on which I had carried out a lot of work. It was pointed out that in those studies every mutant appeared to be different, whereas all the reported monkeypox 'white pock mutants' were identical, and, like the original 'whitepox' isolates, indistinguishable from variola virus. Arrangements were made for a virologist from the Moscow laboratory to work on the problem in the other WHO Collaborating Centre on Smallpox and Other Related Infections, in Atlanta, Georgia.