China is reporting a sudden surge of patients infected with H7N9 bird flu, just days ahead of the start of the Chinese New Year holiday, raising concerns among infectious diseases experts that the holiday may provide the perfect opportunity for the emerging flu strain to spread even further.

The 40-day-long Lunar New Year holiday is considered the world's largest annual human migration, a time when many urban Chinese head home to families in rural hometowns.

An estimated 3.62 billion trips are expected to occur over the 40 days, packing the country's rails, roads and planes.

On Thursday, one day ahead of the holiday’s start, the World Health Organization announced that 15 new human cases of H7N9 bird flu had been diagnosed in China. Of those illnesses, 13 are currently in critical or serious condition.

Since the first H7N9 cases were reported last spring near Shanghai, there have been more than 250 infections; nearly a quarter of those patients have since died.

In the first four weeks of 2014 alone, there have already been 20 H7N9 deaths in China, according to the official Xinhua News Agency. A handful of cases have also been diagnosed in Hong Kong and Taiwan, though all those patients had recently travelled to China.

Most of the human cases have been linked to contact with live poultry. There have been some clusters of cases in families, but the WHO says that so far, there is no evidence of sustained human-to-human transmission.

Nevertheless, some experts are concerned are worried, as the New Year holiday is typically a peak time for chicken sales, with many travellers carrying live birds to their families back home.

"If there is a risk of human-to-human transmission we could easily see large outbreaks happening,” says the WHO Representative in China, Dr. Bernhard Schwartlander.

Gregory Hartl, a WHO spokesman in Geneva, has said his organization is ascribing this month’s spike in human cases to the fact that it's winter -- a season when all forms of the flu spread easily.

He said while there are concerns that the holiday migration could give rise to more infections, it’s also possible there could be no surge in new cases.

Earlier this week, authorities in eastern China announced a ban on live poultry sales. Hong Kong also suspended live chicken sales for three weeks, after poultry imported from the Chinese mainland tested positive for the H7N9 virus.

Hong Kong authorities are also culling 20,000 birds, mostly chickens, at the territory's wholesale market.

The WHO advises travellers to countries with known H7N9 outbreaks to avoid poultry farms, or contact with animals in live bird markets, and urged affected countries to closely monitor infections.

With reports from CTV’s Janis Mackey Frayer in Beijing and The Associated Press