The usual method of isolation of variola and other poxviruses throughout the campaign was inoculation on the chorioallantoic membrane of developing chick embryos. Variola virus produced small white pocks, standard strains of vaccinia virus produced large white pocks but some strains ('rabbitpox' and 'neurovaccinia' viruses) produced pocks with a haemorrhagic, ulcerated centre, and monkeypox virus produced small pocks with an ulcerated centre. In 1958, I had shown that a large number of white pocks, each different in size, could be isolated from rabbitpox virus and that pairs of these would recombine to yield wild-type rabbitpox virus when inoculated into single Hela cells (Fenner, 1959).

One problem that had worried us during the eradication program, but was not finally solved until after the declaration of eradication, was the isolation of what were called 'whitepox' or 'wild whitepox' viruses from several species of African wild animals, and later from stocks of monkeypox virus. Three sets of isolations of 'whitepox' viruses have been made. In 1964–65, four orthopoxvirus isolations were made from cynomolgus kidney cells, at the time being used for virus isolation in the National Institute of Public Health in Bilthoven (Gipsen and Kapsenburg, 1966); one was eventually recognized to be monkeypox virus, one was vaccinia, and two others were indistinguishable from variola virus (Gispen and Brand-Saathof, 1972). On two occasions in 1964, material from smallpox patients from Vellore, in India, had been handled on the same bench in the same laboratory, and skilful detective work by Kapsenburg identified the passage transfer of cell cultures during which contamination could have occurred.

The second set comprised four strains of virus, isolated by workers at the Moscow Research Institute for Viral Preparations in Moscow, one of the two WHO Collaborating Centres for Viral Diagnosis, from the tissues of a chimpanzee, a monkey, a squirrel and a multimammate rat shot in Zaire between 1971 and 1975 (Marennikova et al., 1972). The third set were viruses that produced white pocks which had been recovered from stocks of monkeypox virus that had been maintained for several years in the Moscow laboratories (Marennikova et al., 1979); all these isolates were identical and indistinguishable from those of the second set.

Seeking an explanation, Marennikova postulated that the third set were white variants of monkeypox virus, a virus which had a wide host range and was known to occur in Zaire. However, investigations by Keith Dumbell in London showed that all the 'whitepox' viruses isolated in the Moscow laboratories resembled Indian and not African strains of variola virus, and studies by Jim Nakano at CDC, Atlanta, confirmed these results. In relation to the third set, other studies showed that monkeypox virus, like other orthopoxviruses that produced pocks with a haemorrhagic centre, did produce white pock mutants, but, as with rabbitpox, these differed from one another and resembled monkeypox and not variola virus in most biological tests.

Clearly, it was important to clarify the situation, so one of Marennikova's staff was despatched to the Atlanta laboratory to see if she could reproduce her results there. She was unable to do this using material supplied by the Atlanta laboratory, so she was given permission to use one of the 'monkeypox' strains that she had brought from Moscow, and with this she was able to reproduce her earlier results (Esposito et al., 1985). These investigations demonstrated that all the 'whitepox' viruses, whether isolated from animals from Zaire or recovered from monkeypox virus strains from the Moscow laboratories, were laboratory contaminants, a conclusion with which Marennikova finally agreed.