Taken from: http://www.health.gov.on.ca/en/public/programs/emu/pan_flu/
What is an influenza pandemic?
An influenza pandemic is a flu outbreak distinguished from seasonal influenza by its scope and seriousness. It becomes a worldwide epidemic, or pandemic, when a disease spreads easily and rapidly through many countries and regions of the world and affects a large percentage of the population where it spreads.
How does an influenza pandemic start?
The viruses that cause ordinary/seasonal influenza ? or ?flu? ? are constantly changing. An influenza pandemic starts when a new strain of flu virus emerges, and is different from common strains. Because people have no immunity to the new virus, it can spread quickly and infect hundreds of thousands of people. Influenza pandemic strains can develop when an animal or bird virus mixes with a human virus to form a new virus.
With the H1N1, they are estimating that 35% of the population will be infected, compared with 10% for the seasonal flu. It is a new strain that the general population has no natural immunity to, thus the higher infection rates. Generally, influenza strains result in 2000 to 8000 deaths annually; with H1N1 we are looking at 80 deaths in 2009 [current of October 15th].
The main threat of H1N1 is that we have no natural immunity to this strain, thus the higher infection rates, and also why we are seeing deaths in different population categories [they are now saying H1N1 is higher in women 18-60 years of age]; adults 18-65 years of age are at a higher risk of infection and serious symptoms. Children are not at a high risk for serious illness/outcome, but are a main source of spreading the virus to more vulnerable groups given the nature of school-recreation-home routines.
Basic moral of the story: If you feel sick, STAY HOME. I cannot stress this enough. Too many people are shrugging off symptoms, however mild, and are continuing their routine - infecting others in their workplace, etc. Many places are offering guidelines and extended sick leave to encourage their employees to stay home.