A teenager was hospitalized and in critical condition after contracting bird flu, in what Canadian officials said on Tuesday was the country's first known case of the virus being transmitted to a human.

Health officials in British Columbia were investigating how the teenager came down with the disease despite having no known links to poultry farms, the most common sites where bird flu has been detected in Canada.

The virus, called H5N1, is a low risk to the public, health officials said, because it does not spread easily between people. But prolonged outbreaks, such as a cluster in U.S. dairy cattle can increase the chances of the virus mutating and spreading.

There have been 46 cases of people contracting the virus in the United States, according to the Centers for Disease Control and Prevention, nearly all of them farm workers. Most cases have been mild.

About 36 people who came into contact with the young person in Canada have tested negative for the virus and were offered a preventative course of antiviral medication, Dr. Bonnie Henry, British Columbia's health officer, said at a news conference on Tuesday. Pets also tested negative.

Bird flu, or avian influenza, has not broken out in Canadian dairy farms, but at least two dozen poultry farms have detected the virus since October 2024, officials said.

Officials are making painstaking efforts to track down the source of the viral exposure in British Columbia.

"There is a very real possibility that we may not ever determine the source, but at this point, we have a number of leads that we're following," Dr. Henry said. "We will be tracking down everyone because this is such a rare event."

Officials would not disclose the age or gender of the teenager, but said the person was in critical condition and receiving treatment at the B.C. Children's Hospital in Vancouver after being admitted on Friday.

About a week before that, the teenager went to a hospital emergency room complaining of respiratory symptoms, and had pink eye, fever and a cough. The teenager was sent home at the time and did not attend school before being admitted to the hospital, Dr. Henry said.

There has only been one other case, in 2014, of bird flu in a Canadian citizen. Experts say they believe that person contracted the virus during a trip to China, where H5N1 was first discovered about two decades ago. That Canadian later died.

British Columbia has warned members of the public to avoid handling dead birds or allowing pets near them.

Poultry farmers were also warned to take caution.

"People who are raising ducks and chickens and other domestic fowl are advised to keep their flocks indoors, as these birds are spreading their avian flu viruses up and down North America," said Dr. Brian Ward, a professor at McGill University in Montreal who researches infectious diseases.