Halting the spread of a new virus that has killed hundreds in China is difficult in part because important details about the illness and how it spreads are still unknown. China orchestrated history’s largest anti-viral campaign by blocking 50 million people from leaving Wuhan and nearby cities where the outbreak has been concentrated. Some places are discouraging people from even leaving their apartments, and businesses and offices are closed indefinitely. Foreign governments have rushed to get their citizens out of China while also restricting visitors from the country. And countries’ efforts to hold people in quarantine to stop a possible pandemic have not always been smooth.