A spate of bird flu cases since the beginning of the year in China has experts watching closely as millions of people and poultry are on the move ahead of the Lunar New Year holiday, the world’s largest annual human migration.

China has reported more than 50 H7N9 infections in 2014 after the strain jumped from birds to people for the first time last year. The virus remains hard to catch and most cases have been linked to contact with poultry, but scientists worry that could change if it mutates into a form that allows it to spread easily among people.

For those who track influenza, the holiday, which begins Jan. 31, is always worrying because it comes during the winter months when flu typically rages. Add that to hundreds of millions of people — and often birds — crammed together on buses and other forms of transportation going home, and it’s always a bit of a gamble. China estimates 3.6 billion trips will be taken over the holiday season.

The first H7N9 cases were reported in late March near Shanghai, and more than 200 others have since been identified, including some 50 deaths. A 31-year-old doctor became one of the latest fatalities, raising fears he may have been infected at the hospital where he worked, but none of his patients or other close contacts have reported flu symptoms, authorities said.