Health officials are expressing concern over the H7N9 strain of avian influenza, which recently has infected a small number of people in China.

The World Health Organization (WHO) issued a Global Alert and Response notice on April 1, noting Chinese health officials had confirmed three cases of the virus in humans. Previously, the H7N9 strain was only found in birds.

According to an Associated Press article, this strain can move through poultry flocks without causing any apparent illness, potentially increasing the possibility of humans or other livestock being exposed.

Changes in the virus have made it more capable of infecting pigs, according to the article, further raising concern over transmission to humans.

Quoted in the article, Richard Webby, director of a World Health Organization flu center at St. Jude Children’s Research Hospital, says there is some concern, but at this stage the H7N9 virus is unlikely to cause a global pandemic.