There are many different subtypes of type A influenza virus. These subtypes differ because of certain proteins on the surface of the virus. Infection with certain avian influenza A virus caused by some H5 and H7 strains can cause widespread disease and death among some species of domestic birds. Since 1997, several outbreaks of H5 and H7 influenza have occurred only among poultry in the United States. In some parts of the world, the H5N1 avian influenza virus and H7N9 virus have, however, resulted in bird flu in humans. Bird flu has a high death rate in humans, possibly because people have little immune system protection against avian influenza viruses. Most cases of bird flu infection in people have occurred as a result of contact with infected birds. Infection also may result from contact with contaminated surfaces, from eating undercooked poultry or eggs from infected birds or through contact with a person who has bird flu, which in fact rarely occurs. Researchers look at a number of factors including the genetic markers of each virus to help determine the risk for human infection.