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The Centers for Disease Control and Prevention has issued a new travel warning about bird flu in China.

It says the H7N9 strain of bird flu appears to be unusually active again and advises people traveling to China should stay away from chickens and poultry markets.

“Chinese health authorities have confirmed 120 new human cases of avian influenza A (H7N9) since September 2016, with 37 deaths,” the CDC says in the low-level travel alert.

“Most of these patients reported exposure to live poultry or poultry markets. Infections have been reported in the provinces of Jiangsu, Fujian, and Guangdong, as well as the Macau and Hong Kong Special Administrative Regions.”

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Related: Watch Out for H7N9 Bird Flu, WHO Says

H7N9 is one of several strains of bird flu that officials are watching because they have the potential to cause a human pandemic.

So far, H7N9 doesn’t seem to infect people easily and people who are infected do not seem to spread it to others much, if at all. But influenza viruses change quickly and unpredictably and if one starts passing easily from one person to another, it could spread.

“CDC advises people traveling to China to avoid contact with poultry (including poultry markets and farms), birds, and their droppings and to avoid eating undercooked poultry,” the CDC said.

Related: H7N9 Bird Flu Spreads Like Ordinary Flu

“There are no recommendations against travel to China.”

Influenza viruses can spread from birds and other animals to people and back again. They can mutate and swap genetic material in the process, becoming more or less contagious and more or less dangerous.

H7N9 has been on the radar since spring of 2013, when China began reporting infections with the virus in people.

Related: H7N9 has cost billions

“Early symptoms are similar to those of seasonal flu and may include fever, cough, sore throat, muscle aches and fatigue, loss of appetite, and runny or stuffy nose. However, infection with this virus often causes severe respiratory illness and, in some cases, death,” the CDC said.

“Clinicians should consider the possibility of avian influenza A (H7N9) virus infection in people presenting with respiratory illness within 10 days of travel to China, particularly if the patient reports exposure to birds or poultry markets.”