A pandemic is a global outbreak of a virus for which there is little to no immunity in the human population. It is defined by the spread of disease, not by its ability to cause death. The 20th century saw three pandemics and the 21st century experienced one in 2009-2010.

Flu viruses are constantly evolving, changing into new strains and types, and this dictates the need to adapt the treatments and vaccines every year in order to keep up with the changes. There are three types of flu viruses; types A and B cause epidemics or widespread outbreaks of infection whereas type C causes mild respiratory illness and is not thought to cause epidemics. Type A viruses are found in many animals, including ducks, chickens, pigs, whales and others. Type B circulates widely in humans whereas Type C has been found in humans, pigs and dogs.

For example, Type A viruses are divided into subtypes which include H1N1, H1N2, and H3N2. In order for there to be a worldwide pandemic, there are certain factors that must be met; a new subtype of Type A virus must be introduced to the human population that can cause serious illness and spread easily from person-to-person in a sustained manner. This was the case with the H1N1 Flu that the World Health Organization (WHO) declared a global pandemic from 2009-2010.

The differences between seasonal and pandemic influenza include:

• Seasonal flu outbreaks follow predictable seasonal patterns (aka flu season) whereas pandemic flu outbreaks rarely occur
• Seasonal flu allows for immunity to build up due to previous exposure; pandemic flu has little or no pre-existing immunity
• Healthy adults are not at as high a risk for complications due to seasonal flu; Pandemic flu puts everyone is at risk, often having more severe symptoms and greater potential for complications
• Healthy systems can usually meet public needs for seasonal influenza where they may get overwhelmed by pandemic flu
• Seasonal vaccines and treatments have been developed, refined and stockpiled over years; Pandemics would require new vaccines, treatments and antivirals to be developed quickly
• The death toll for a pandemic is much higher than the seasonal average. This is determined by the number of infected people, the virulence of the virus, the vulnerability of the affected population and the effectiveness of preventative measures
• Seasonal flu causes a mild to moderate impact on society whereas pandemic flu causes a major social impact