As of 10 a.m. today, planes and trains leaving the city were to be canceled, and buses, subways and ferries within it suspended. Residents were urged not to leave.

The move could upend the plans of millions of Chinese citizens, who travel in huge numbers during the Lunar New Year holiday that officially begins on Friday. Experts have warned that the virus would proliferate during the mass travel period.

World Health Organization: The group postponed a decision on whether the outbreak constitutes a “public health emergency of international concern.”

Context: The disease began in a seafood and poultry market in the city of 11 million, but is unclear from what kind of animal.