Where it infects:
Typical influenza viruses infect the cells lining the main airway and nasal tract, according to Earl Brown, a virologist at the University of Ottawa.
The virus also infects the airway, but in some individuals, it continues to move down and infect the lungs. It's "very unusual" for seasonal flu viruses to infect lungs, Dr. Brown said. The H1N1 virus causes lung infection much more often and "more completely," he said, which is why some people have ended up in intensive care units or even dying from complications of the virus.
The Globe on H1N1
Cough, sore throat, fever, headache, muscle ache, loss of appetite, runny nose, joint pain and fatigue are common symptoms of seasonal flu. Some people may also experience nausea, vomiting and diarrhea, although this tends to be more common in children.
H1N1 Many symptoms of this virus are similar to seasonal flu, but there are some warning signs that may indicate a case that needs immediate medical attention. Common symptoms include sore throat, cough, fever, muscle aches, headache, loss of appetite and runny nose, according to the Public Health Agency of Canada. Health experts say more people with H1N1 appear to experience vomiting and diarrhea than typical seasonal influenza cases. Symptoms that warrant immediate medical attention include any rapid breath, shortness of breath or difficulty breathing, as well as if a person's complexion is grey or lips appear blue. Another warning sign is if someone is extremely lethargic, isn't making sense or appears generally out of it.
Although seasonal flu can affect people of any age and occasionally cause complications, the vast majority of the severe cases and deaths occur in the elderly.
Unlike seasonal flu, the virus is hitting younger people particularly hard, as well as people with certain health conditions. The Public Health Agency of Canada has identified groups it considers most at risk: Children under age 5; pregnant women; those with chronic conditions, such as asthma, heart or kidney disease, chronic lung disease, liver disease, suppressed immune systems, neurological disorders, blood disorders and severe obesity.