Using his model, Ronald Ross showed that it wasn’t necessary to remove every mosquito to bring the disease under control: 

Destroy enoughmosquitoes, and people infected with the parasite would recover before they were bitten enough times for the infection to continue at the same level. Therefore, over time, the disease would fall into decline. In other words, the infection had a threshold, with outbreaks on one side and elimination on the other.

Ross’s work, which won him a Nobel Prize in 1902 and a knighthood in 1911, set the stage for a new mathematical way of thinking about disease outbreaks from bubonic plague to influenza. His insight influenced vaccine policy through the concept of ‘herd immunity’: vaccinate a sufficient proportion of the population, and the disease will fail to take off. It means that vaccination can work even if a few people are left unprotected. Although the specific control measure is different – giving vaccines rather than removing mosquitoes – the principle is the same. As long as we remove enough links in the chain of events that generate infections, the disease will die out. It isn’t necessary to vaccinate everyone, or remove every mosquito; if we reach the critical threshold, the infection will struggle to cause outbreaks in the population.