Ross calculated that even if there were 48,000 mosquitoes in a
village that contained someone infected with malaria, it might only
result in one additional human caseWhen he wrote up the analysis in his 1910 book The Prevention of
Malaria, Ross acknowledged that his readers might not follow all of
his calculations. Still, he believed that they would be able to
appreciate the implications. ‘The reader should make a careful study
of those ideas,’ he wrote, ‘and will, I think, have little difficulty in
understanding them, though he may have forgotten most of his
mathematics’. Keeping with the mathematical theme, he called his
discovery the ‘mosquito theorem’.
The analysis showed how malaria could be controlled, but it also
included a much deeper insight, which would revolutionise how we
look at contagion. As Ross saw it, there were two ways to approach
disease analysis. Let’s call them ‘descriptive’ and ‘mechanistic’
methods. In Ross’s era, most studies used descriptive reasoning.
This involved starting with real-life data and working backwards to
identify predictable patterns. Take William Farr’s analysis of a London
smallpox outbreak in the late 1830s. A government statistician, Farr
had noticed that the epidemic grew rapidly at first, but eventually this
growth slowed until the outbreak peaked, then started to decline. This
decline was almost a mirror image of the growth phase. Farr plotted a
curve through case data to capture the general shape; when another
outbreak started in 1840, he found it followed much the same path.