This month, two independent cases of bird flu were detected in North American children without any known exposure to infected animals, raising concerns that the H5N1 virus that causes it is inching closer to evolving in a way that allows it to spread between humans.

Since April, 55 H5N1 cases have been reported in humans, and all but three have occurred in farmworkers in close contact with dairy cows or poultry, which the virus is infecting in droves. But health officials have not been able to determine the source of three cases in humans, raising questions about whether there is low-level community spread happening.

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On Nov. 9, government officials in British Columbia reported that a teenager tested positive for H5N1 with no known exposure to an infected animal. Last week, a child in the Bay Area also tested positive for bird flu without any known exposures. These two cases follow a third infection in Missouri reported in September, for which health officials were unable to determine the origins of the infection after an extensive investigation.

“The big takeaway is that there is more community spread than is being detected,” said Dr. Abraar Karan, an infectious disease physician at Stanford University. “When you can’t figure out where the infection came from, that raises a lot of red flags.”

Colorized transmission electron micrograph of Avian influenza A H5N1 viruses (seen in gold) grown in MDCK cells (seen in green). (Smith Collection/Gado/Getty Images)Without exposure to farm animals, it’s possible these children could have become infected after coming into contact with a wild bird infected with the virus. Another possibility is that they could have come into contact with a domesticated animal that had the virus. However, in the Canadian teen's case, all of the pets they came into contact with tested negative, said Bonnie Henry, a public health officer for the province of British Columbia in Victoria, Canada, during a press conference.

“There is a very real possibility that we may not ever determine the source,” Henry said.

In another press conference hosted today, Henry........

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