An influenza pandemic is a global outbreak of a new influenza A virus that is very different from current and recently circulating human seasonal influenza A viruses. Pandemics happen when new (novel) influenza A viruses emerge which are able to infect people easily and spread from person to person in an efficient and sustained way. Because the virus is new to humans, very few people will have immunity against the pandemic virus, and a vaccine might not be widely available. The new virus will make a lot of people sick. How sick people get will depend on the characteristics of the virus, whether or not people have any immunity to that virus, and the health and age of the person being infected. With seasonal flu, for example, certain chronic health conditions are known to make those people more susceptible to serious flu infections. The risk factors associated with seasonal flu can be found at “People at High Risk of Developing Flu–Related Complications.” Influenza pandemics are uncommon; only occurred during the 20th century.