The large spatial scale, geographical overlap, and similarities in transmission mode between the 1918 H1N1 influenza and 2020 SARS-CoV-2 pandemics together provide a novel opportunity to investigate relationships between transmission of two different diseases in the same location. In their new study titled "Estimating R0 from early exponential growth: Parallels between 1918 influenza and 2020 SARS-CoV-2 pandemics", Grant Foster, Dr. Tad Dallas and their collaborators estimated the basic reproductive number of both disease outbreaks in a common set of 43 cities in the United States. They found that despite occurring in the same locations, there is a surprising lack of concordance both between reproductive number estimates for the 1918 influenza and COVID-19 outbreaks. The diseases also exhibit opposing patterns in the effects of both population size and epidemic. For example, epidemics that started later tended to be less severe for COVID-19, while influenza epidemics exhibited an opposite pattern. A fascinating study!