Adam Kucharski studies how diseases spread, but he’s not handling viruses in the lab or treating sick people in the hospital. He’s a mathematician at the London School of Hygiene & Tropical Medicine, and he uses math to understand outbreaks of diseases like Ebola, SARS, influenza and now Covid-19. His goal is to design better ways to control outbreaks.

In an eerie coincidence, he wrote a book called “The Rules of Contagion,” before the current outbreak, which has been published in Britain and will be released in September in the United States. In it he talks about the math of contagion involving not only physical diseases, but also ideas, rumors and even financial crises.

In a recent experiment for the 100th anniversary of the 1918 flu, he worked with another mathematician and BBC presenter, Hannah Fry, of University College London, and collaborators at the University of Cambridge, to create a documentary, “Contagion: The BBC Four Pandemic,” using a phone app to track social contacts and map how an infection might spread.

The news of coronavirus epidemics around the world involves a flood of numbers that are a challenge for any nonscientist to digest. I asked Dr. Kucharski to help us navigate some of these numbers, and to tell us which ones we should pay attention to. We talked on the phone and corresponded by email this week. This is an edited version of our back and forth.