Health officials are awaiting test results to determine whether the deadly Ebola virus has arrived in Canada.

But on Tuesday, doctors ruled out malaria or meningitis as the mysterious illness that's left a woman in serious condition in Hamilton, Ont.

The unidentified woman who arrived in Canada from Congo on Saturday was hospitalized Sunday night with symptoms common to Ebola and other hemorrhagic fevers prevalent in central Africa.

At first doctors thought she might have malaria or meningitis. But tests on Tuesday made those possibilities "less and less likely," said Dr. Mark Loeb, an expert in infectious diseases who works at the hospital.

Loeb said the woman, who is being held in isolation, isn't showing any end-stage symptoms of Ebola, such as bleeding from the eyes or nose.

But he said she is in serious condition, and is being held in an isolation room where negative pressure prevents the release of airborne particles.

The woman's blood and fluid samples are now being tested at the infectious diseases laboratory in Winnipeg, and at the Centers for Disease Control in Atlanta.

If confirmed, it would be the first case of Ebola in North America.

Risk of outbreak is low

Health officials are downplaying the risk to the public, saying the likelihood of a large-scale outbreak is low because hemorrhagic fevers can only be transmitted by close contact with bodily fluids.

"We're talking about exposure to mucous membranes with blood or saliva or those kind of secretions," said Loeb.

Officials say they only know of two people who have had that kind of close contact with the woman, and those two people have shown no symptoms of the disease.

Still, officials aren't taking any chances.

That's why Air Canada has given Health Canada a list of names of the 39 passengers and crew on the Saturday flight from Newark, N.J., to Toronto, which the infected woman boarded after arriving from the Congo.

Public health authorities say there's no concern about her earlier flight from Congo to New York. "We're satisfied now that this person did not become ill until after arrival in Hamilton," says Dr. Monir Taha.

Officials are also keeping an eye on the 10 hospital staffers who initially came into contact with the woman. "We're looking at who had exposure to this patient ... before the possibility that viral hemorrhagic fever was entertained," said Loeb.

In Ottawa, Immigration Minister Elinor Caplan rejected suggestions that Canada should step up its screening efforts, noting more than 200 million people cross the border annually.

"We can't shrink-wrap our borders," she said, adding that Canadians travelling abroad are not required to submit to health checks on their return either, a civil right valued by some people.

Earlier Tuesday, Ontario's health minister called on the government to review immigration screening procedures and enact new measures to protect public safety.

An outbreak of Ebola last year in Uganda killed about 250 people. There is no cure.