An influenza pandemic occurs when a novel influenza A virus spreads from person to person around the world and affects a very large number of people. A novel flu A virus might cause a flu pandemic if the novel virus can spread efficiently from person to person (instead of just from animals to people or from one person to another, but then no further). Another condition for a pandemic to occur is that most people will not have immunity to the new virus. People can have some immunity to novel or pandemic flu A viruses that are similar to flu A viruses they’ve been infected by in the past. If a new flu A virus is very different from earlier flu A viruses, it may be more likely to cause a pandemic. Influenza B viruses do not cause influenza pandemics.