The development of the virus concept as reflected in corpora of studies on individual pathogens. 5. Smallpox and the evolution of ideas on acute (viral) infections.

IN 1806, Thomas Jefferson was in his second term of office as the third President of the United States. From his neo-classical mansion of Monticello in Virginia, Jefferson wrote on 14 May a letter to Edward Jenner, congratulating him on his discovery of vaccination which Jefferson helped to promote in the United States. After comparing Jenner's discovery favourably with William Harvey's discovery of the circulation of the blood, Jefferson continued in somewhat purple prose: "You have erased from the calendar of human afflictions one of its greatest. Yours is the comfortable reflection that mankind can never forget that you have lived; future nations will know by history only that the loathsome small-pox has existed, and by you has been extirpated ... .".I The optimism voiced by Jefferson proved to be highly premature. In the last decade, the vast resources of the World Health Organization have been brought to bear on the problem of world-wide eradication of variola; even so, the last pockets of infection have proved more persistent than expected, and optimistic estimates far more recent than Jefferson's remain so far unfulfilled.2 On the other hand, in the absence of an animal carrier of the virus of smallpox, complete eradication may be obtained with the help of vaccination, while bubonic plague, carried by a number of rodents, remains endemic over large areas of the globe, including North America. Thus twentieth-century public health authorities still face some degree of challenge from the two major scourges of mankind, which through massive epidemic outbreaks exercised natural population control throughout medieval and early modem history. *This work was made posible by a grant from the Weilcome Trust. **Lise Wilkinson, Cand. Pharm., Mag. Scient., Department ofVirology, Royal Postgraduate Modical School, Du Cane Road, London W.12. 1 The letter is quoted in its entirety by Baron: John Baron, The life of Edward Jenner, M.D., London, Henry Colburn, 1838, vol. 2, pp. 94-95. 'The imminent complete eradication of smallpox was predicted in 1976, after ten years of the "intensified global eradication programme" launched by the World Health Organization in January 1976. D. A. Henderson wrote: "The world may have seen its last case of the most devastating disease in human history. Smallpox... is mang its last stand in two remote areas of Ethiopia, one in the desert and one in the mountains. ." (D. A. Henderson, 'The eradication of smallpox', Scient. Am., 1976, 235 (4): 25-33, p. 25). Since then, war in Ethiopia has added to the problems faced by the WHO teams, and the target of complete global eradication remains in question; on 19 April 1978, the WHO announced the offer of a global reward for any report of a case of smallpox.