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Stage one

People can become ill if the flu virus enters their nose, eyes or throat after an infected person coughs or sneezes. They can also become ill after touching an infected surface, such as a doorknob, keyboard or countertop, and then touching their mouth or nose.

The virus begins to grow, and symptoms can start to develop 24 to 48 hours after initial infection. However, it can take up to a week for symptoms to develop. People seem to become contagious one day before the onset of flu-like symptoms, says the Public Health Agency of Canada.

Stage two

Doctors say most people can't properly distinguish between a cold and the flu, but that influenza viruses come with several telltale symptoms. Similar to seasonal flu viruses, H1N1 symptoms include sore throat, cough, headache, loss of appetite, fatigue and aches and pains.

Fever, a typical symptom of seasonal flu, isn't appearing in all H1N1 cases, said Earl Brown, a virologist at the University of Ottawa. But more people with H1N1 seem to be experiencing vomiting and diarrhea than they would with a regular seasonal flu virus, Dr. Brown said.

People with H1N1 will typically spend from two to four days in bed. But serious symptoms require medical attention, including shortness of breath and extreme lethargy or being “out of it,” said Michael Gardam, director of infectious diseases at the Ontario Agency for Health Protection and Promotion.

Stage three

The average person with the H1N1 virus will be back on their feet after about a week. However, it could take from two to three weeks before they feel 100 per cent, Dr. Brown said. People who are infected with H1N1 may remain contagious slightly longer than those with seasonal flu.

Public Health says people infected with H1N1 remain contagious for about a week after they develop symptoms.

However, some people's symptoms can grow worse after they initially begin to improve, which could be a signal of a secondary bacterial pneumonia. Although different from viral pneumonia, one of the rare and serious complications appearing in a small number of people with H1N1, bacterial pneumonia is very serious and requires medical intervention.

Warning signs

People who get more severe illness and complications from the H1N1 pandemic virus will experience worsening symptoms over the first few days, said Anand Kumar, an intensive-care specialist at Winnipeg Health Sciences Centre.

Coughing up blood or turning blue are obvious danger signs. But another key warning sign is shortness of breath. Parents whose children are ill should monitor them to see whether they are breathing rapidly. “I sure as hell wouldn't be waiting around at home to see how I do once I'm short of breath,” Dr. Kumar said. “It can be very rapid once it goes to your lungs.”

How the virus kills

This virus, much more so than seasonal influenza virus, tends to attach to the cells within the lower respiratory tract where it can cause severe infection, experts say. This infection of the lungs can trigger a cascade of inflammatory responses. Once the virus attaches itself to the deep respiratory cells, it starts to damage the lungs, which start to fill with fluid, and the person can't get enough oxygen. There is evidence the virus can go to the heart in rare circumstances, which can lead to rhythm disturbances and death.