A Canadian teen’s bird flu infection is raising questions about the transmission of the virus. As the teen slipped into critical condition on Tuesday, Nov. 12, with a life-threatening lung condition, doctors are still working to figure out how the infection occurred with no known exposure to infected people or animals.

The individual, who has not been identified by name or gender, reportedly never went on a farm and all pets in their home have tested negative for H5N1. Meanwhile, contact tracing is ongoing among 35 to 40 people including medical workers with known contact with the teen.

This is reportedly the first case of bird flu in Canada since 2014. It is similar to a case of a Missouri teen with no known prior exposure to H5N1 earlier this year.

U.S. officials are reportedly in contact with Canadian health authorities, and the U.S. Centers for Disease Control and Prevention (CDC) offered assistance to Canada if needed.

The CDC reported that 45 of the 46 confirmed bird flu cases in the United States have come from farm workers exposed to the infection.

The Canadian teen’s case comes as a U.S. Congressional report urged the United States to ready for a “pandemic influenza response” before President-elect Donald Trump takes office on Jan. 20. The report pushing for more protective equipment, testing and antiviral treatments as virologists warn that the virus’ transmission may be evolving.

Despite the concerns, the World Health Organization (WHO) still rates the risk of human-to-human transmission of bird flu as “unlikely.”

However, the CDC has expanded bird flu testing to all farmworkers even those without symptoms of the virus, and the U.S. Department of Agriculture has expanded the testing of milk.