Last night, I spent some time with Arin Dutta's "The Effectiveness of Policies to Control a Human Influenza Pandemic: A Literature Review." The overview was prepared for the World Bank. In it, Dutta argues that the key variable in determining the spread of an infection is the "base reproduction rate," defined as the number of secondary infections produced by a primary infection. In other words, if one person has the flu, then on average, the base reproduction rate measures how many people will catch the flu from him. Lowering that rate is the key to pandemic response. And lowering that rate requires things like "forcing or urging people to limit contacts, encouraging hand washing or other personal hygiene, or promoting the use of facemasks." Some of it sounds trivial. But it matters. If the reproduction rate falls beneath 1, "the epidemic usually dies out." But even small variations in the rate can have large impacts, as you see in this chart: