Infectious diseases occur without fail in places where people are crowded together. However, it is impossible to avoid crowded environments at an evacuation shelter when disaster strikes. How, then, should we control the inevitable infectious diseases? Experts say appropriate countermeasures and systems for monitoring signs of outbreaks are crucial. At the time of the Great East Japan Earthquake, experts and a local government infection control team conducted a survey of trends in the outbreak of infectious diseases in Iwate Prefecture and succeeded in minimizing the size of disease clusters. Meanwhile, since the start of the COVID-19 pandemic, measures to combat the spread of infectious diseases in crowded environments have become prevalent throughout society. We must not let this experience go to waste.