The outbreak of the new coronavirus that started in the Chinese city of Wuhan has mobilized medical efforts around the world. The World Health Organization declared the outbreak a “Public Health Emergency of International Concern” on Thursday. By Friday, the disease had killed more than 200 people in China and infected thousands. The fast-spreading virus, which first appeared in central China in December, has been confirmed in more than 20 countries and territories. The list includes Japan, South Korea, Singapore, Thailand, Vietnam and Cambodia. But countries outside of Asia, including the United States, Australia, Canada, Germany, Britain and France, have reported confirmed cases. Most of those who have been infected in other countries had recently visited Wuhan. China has barred travel from Wuhan and other cities in central Hubei province. The ban places a partial quarantine on more than 50 million people.