The origin of major human infections and the crucial role of person-to-person spread

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Abstract
In discussions of the origin of new infectious diseases, prominence is usually given to dramatic infections acquired from animals (Lassa fever, Marburg and Ebola viruses) or from the environment (legionellosis). But these infections do not spread from human to human, and their impact on mankind can never be catastrophic. If a new infectious agent is to pose a major threat to the human species, it will need to kill tens or hundreds of millions of people over a short period of time (a few years), before vaccines and antimicrobial agents can be developed or effective blocks to transmission established. The 1918 influenza pandemic (total deaths about 20 million) almost came into this world-shaking category, but many of the deaths were presumably due to secondary bacterial pneumonia and this was the pre-antibiotic era. A similar airborne pandemic occurring today could spread globally within weeks by air transport and would have a greater impact. Smallpox arose long before the world became one from an infectious disease point of view, and although it caused devastating epidemics it did not have the opportunity to develop into the ‘major threat’ category. A significant proportion of people recovered, less virulent strains of virus (variola minor) appeared, and an effective vaccine not only kept it under control in many continents but finally eliminated it from the world. The plague (Yersinia pestis) influenced the course of history in Asia and Europe [1] but oven in its respiratory form could not in those days be transferred rapidly from continent to continent.