About one third of the North American population may become infected with the H1N1 virus this fall, according to a Canadian microbiologist. The disease has already infected thousands across Canada and is linked to at least 41 deaths.

"What we think will happen in the fall in North America is a more complete wave, in which 30-40 per cent of people become infected over a 12-16 week period," said Allison McGeer, microbiologist and infectious disease expert at Mount Sinai Hospital.

The virus has infected nearly 4,000 people in Ontario alone since it first spread from Mexico City last spring. The World Health Organization declared a global flu pandemic last month -- the first such pandemic in four decades.

Last week, 200 scientists gathered in Toronto to discuss ways to curb the number of H1N1 infections.

This fall, experts are predicting the virus will change -- not necessarily into a more deadly strain, but into a second wave of the pandemic that will affect significantly more people.

"The nature of flu viruses is that they change all the time, however, we are not expecting there to be a major change in the cause or severity of symptoms with H1N1," said McGeer.

H1N1 is also expected to become a seasonal influenza strain once the pandemic is over, something which is characteristic of pandemic viruses.

Dr. Earl Brown, a flu expert at The University of Ottawa, told CTV News Channel on Thursday that the government and hospitals are doing everything they can to ensure they have enough supplies for the fall influx of H1N1 patients.

"Hospitals will have to stock up on medicine for people who are on respirators, as well as, buy extra respirators. They are gearing up for a full blown flu season," said Brown.

Brown said an H1N1 vaccine is slated to be released in the late fall, likely November.

The Canadian government has a contract with the international vaccine manufacturing company, GlaxoSmithKline, to develop the drug from its Quebec plant.

The United States will also roll out their version of the H1N1 vaccine in October.

While the contents of Canadian vaccine will be the same as the U.S. vaccine -- determined by the World Health Organization - there may be slight variations in the make up due to different manufacturers.

As for how the vaccine will be administered to the public, McGeer said the government will likely target young adults with chronic illnesses, such as diabetes, asthma and chronic lung disease, as well as pregnant women.

"It may mean that the first vaccines off the plant will be administered by public health to target the most vulnerable people, presuming there will be a limited amount of the vaccine, but we really don't know exactly at this point," said McGeer.

It is expected that either one or two doses of the vaccine will be available for each person.

"Hopefully the structure of the virus won't change too much and the vaccine will work just fine," said Brown.

Eventually the H1N1 vaccine will be incorporated into the regular flu vaccine, however, there wasn't enough time to incorporate the two vaccines for this year's flu season due to the amount of laboratory testing required.

This month, a new flu virus was discovered in at least two Saskatchewan hog farm workers, but was not connected to H1N1.

McGeer told CTV.ca she thinks the likelihood of the new virus turning into a second full- blown pandemic is highly unlikely.

"It's important to remember is that any new successful flu strain, which develops out of no where and turns into a pandemic, such as H1N1, appears only once every 30 to 40 years. So the probability there is going to be a second one this year is not something we should be losing sleep over," said McGeer.

Dr. Frank Plummer, director of the National Microbiology Laboratory in Winnipeg, concurs with McGeer's sentiments.

"So we've picked up this novel virus. We have to react very aggressively to it. But from what we know now, I think it's probably going to end with those three cases and everything will be fine," Plummer told the Canadian Press on Wednesday.