With 46 U.S. cases and alarming signs of the virus crossing species, experts warn of unpredictable risks if human-to-human transmission were to emerge.

A teenager in British Columbia was hospitalized in critical condition with the disease, and officials were working to find out how the person was exposed to the virus.

Initial testing had indicated the teenager’s infection was from bird flu and was confirmed in further testing, the Public Health Agency of Canada said in a statement. The teen has been hospitalized in Vancouver since Friday and was reported in critical condition on Tuesday.