Influenza that spreads so easily that it infects many people in many different places is called pandemic influenza. Pandemic influenza may cause mild illness or serious illness leading to death. Public health authorities around the world have been thinking about ways to use vaccines and drugs to prevent the spread of pandemic influenza. There are 2 kinds of influenza that authorities are worried about. The first kind, H1N1 influenza or “swine flu,” spreads easily in humans but is generally less dangerous. The second kind, H5N1 influenza or “bird flu,” has so far not spread that easily in humans. However, the second kind seems more dangerous, and people think it soon may spread easily in humans.