The spread of a mysterious respiratory virus has prompted the authorities to limit travel in cities in China, including Wuhan, where the disease was first found last month. It has since spread across the nation and to at least 10 other countries.

The outbreak intensified just before the start of the Lunar New Year holiday on Friday, when hundreds of millions of people were traveling across China. Many people who work and live in Wuhan and the surrounding area are now spending time with their families in other parts of the country. Epidemiologists fear that could make the virus harder to contain.

Here’s a look at the public health crisis in photographs.

Tuesday