People travel with their mobile phones. In January, prior to the lunar new year on January 24th, China began its annual chunyun mass migration. This can involve as many as 3 billion trips, and Wuhan is a central hub in China’s rail and aviation networks. Researchers used real-time mobile phone data from almost 12 million people to look at the spread of the epidemic from Wuhan across China, and the impact of quarantine on infection rates. This could be a useful approach in the future.