Ontario health officials said Friday they believe three people sick in the province have the West Nile virus, which would be Canada's first human cases of the mosquito-borne illness that has killed 24 people in the United States this year.

Colin D'Cunha, the Ontario chief medical officer, said final confirmation would take another two weeks.

Ontario is Canada's most populous province, and is across the border from U.S. states where the virus has been reported.

If the three have the virus, it would document the continuing spread of West Nile throughout North America. Several U.S. states -- including North Dakota, Iowa, Arkansas and South Carolina -- have reported their first human cases this week of a virus that can cause fever, body aches, brain swelling, coma, paralysis or death.

Canadian officials have warned in recent years that the virus, which has been detected in birds and animals in Canada, eventually would spread to the human population. It is transmitted to humans by mosquito bites.

Dr. David McKeown, the medical health officer for the Peel region west of Toronto, said two of the probable cases involved people over 65 from the Mississauga area.

One remains hospitalized in serious condition and another was hospitalized and then released to recover at home, McKeown said.

There was little information immediately available about the third case, but D'Cunha said authorities believe the person became sick in the United States.

According to the U.S. Centers for Disease Control and Prevention, 24 people have died across the country so far this year from the virus, with almost 500 cases reported in more than 20 states.

The virus is most dangerous for children, the elderly and people with weak immune systems. It can cause flu-like symptoms and encephalitis, a potentially fatal brain infection. Most people bitten by an infected bug never get sick.

West Nile is common in Africa and the Middle East. The disease was first detected in the United States in 1999, when seven people died from it in New York.