• A pandemic emerges from an entirely new strain of the influenza virus against which many people may have little or no immunity. Seasonal flu, on the other hand, emerges from slightly changed flu viruses against which many people have some immunity.

• Seasonal influenza strikes in the winter months. Pandemic influenza can strike at any time of year.

• Pandemic influenza has the potential to affect almost every person in the world within a short time frame. This is why pandemics have the potential to scare: In the worst case scenario, people will die not only from complications of the disease but also because hospitals are overcrowded; supplies of antivirals and antibiotics are not sufficient; and because there hasn’t been enough time to make a vaccine. This is what spurs pandemic preparedness efforts. The economic and social impact of a milder pandemic is harder to determine and sometimes hotly debated.