Swine influenza
The changes in the flu virus can cause a flu pandemic. For example, the virus might only change a little bit in about two to three years. After about ten years, most people will have developed antibodies to fight these small changes. But if the virus then changes a lot, there can be a pandemic. Hundreds of millions of people suddenly realize that the antibodies they developed are useless. Even during the Spanish flu pandemic, the first wave of the disease was relatively mild, while the second wave was very deadly.