China decided Thursday to lock down three cities that are home to more than 18 million people in an unprecedented effort to try to contain a deadly new viral illness that has sickened hundreds and spread to other cities and countries in the Lunar New Year travel rush.

And in the capital of Beijing, authorities canceled “major events” indefinitely, including traditional large-scale Lunar New Year celebrations, according to an announcement by the city’s bureau of culture and tourism. It said the move was needed to “execute epidemic prevention and control.”

Police, SWAT teams and paramilitary troops guarded Wuhan’s train station, where metal barriers blocked the entrances at 10 a.m. sharp. Only travelers holding tickets for the last trains were allowed to enter, with those booked for later trains being turned away....