The Rules of Contagion

would turn out to be one of the most important concepts in outbreak
analysis.[38]
The idea in question had originated with George MacDonald, a
malaria researcher based in the Ross Institute at the London School
of Hygiene & Tropical Medicine. In the early 1950s, MacDonald had
refined Ronald Ross’s mosquito model, making it possible to
incorporate real-life data about things like mosquito lifespan and
feeding rates. By tailoring the model to actual scenarios, MacDonald
was able to spot which part of the transmission process was most
vulnerable to control measures. Whereas Ross had focused on the
mosquito larvae that lived in water, MacDonald realised that to tackle
malaria, agencies would be better off targeting the adult mosquitoes.
They were the weakest link in the chain of transmission.[39]
In 1955, the World Health Organization announced plans to
eradicate a disease for the first time. Inspired by MacDonald’s
analysis, they had chosen malaria. Eradication meant getting rid of all
infections globally, something that would eventually prove harder to
achieve than hoped; some mosquitoes became resistant to
pesticides, and control measures targeting mosquitoes were less
effective in some areas than others. As a result, would later shift
its focus to smallpox, eradicating the disease in 1980.[40]
MacDonald’s idea to target adult mosquitos had been a crucial
piece of research, but it wasn’t the one that Bailey had omitted in his
textbook. The truly groundbreaking idea had been nestled in the
appendix of MacDonald’s paper.[41] Almost as an afterthought, he
had proposed a new way of thinking about infections. Rather than
looking at critical mosquito densities, he suggested thinking about
what would happen if a single infectious person arrived in the
population. How many more infections would follow?
Twenty years later, mathematician Klaus Dietz would finally pick up
on the idea in MacDonald’s appendix. In doing so, he would help
bring the theory of epidemics out of its math ematical niche and into
the wider world of public health. Dietz outlined a quantity that would
become known as the ‘reproduction number’, or R for short. R
represented the number of new infections we’d expect a typical
infectious person to generate on average.