A pandemic is a global disease outbreak. A flu pandemic occurs when a new virus emerges for which there is little or no immunity in the human population. Once international spread begins, pandemics are considered unstoppable, and even a mild pandemic flu can ultimately cause millions of deaths worldwide. Last June 2009, the World Health Organization declared that the new, H1N1 virus had become a pandemic flu. More than 70 countries and all 50 states have reported human cases of H1N1 flu. What makes H1N1 special?It’s new, so humans have no resistance to it. Pandemic flu strains are often new viruses that develop when an animal or bird virus mixes with a human virus. The H1N1 virus originally was referred to as “swine flu” because early laboratory tests showed that many of the genes in this virus were similar to those that infect pigs. It turns out that while the H1N1 strain has two genes from flu viruses that circulate in pigs in Europe and Asia, it also contains bird flu genes and human genes. H1N1 is now a human flu virus causing widespread illness.