Test Your Knowledge

There are hundreds of types of flu in nature. They are known to infect humans, birds, horses, dogs, seals and whales. Of the many influenza A viruses that typically infect birds, five are known to also cause disease in humans. These include the following:

• H5N1. The most severe and widespread type of bird flu. It has been confirmed in birds in portions of Asia, Europe and Africa. Since 2003, more than 350 cases have been confirmed by the World Health Organization, though infection may be more widespread. Not all people who are infected show the typical respiratory disease and may not be tested for the virus. Most cases were directly linked to the patient working closely with poultry.
  
  
• H7N2, H7N3 and H7N7. These three viruses have caused infection in the United States (Virginia and New York), the Netherlands and Canada. The viruses caused only mild symptoms and most cases were directly linked to working with poultry. There were also limited reports of disease transmission within the families of poultry workers and one case in which the source could not be determined.
  
  
• H9N2. This strain of influenza has only caused disease in mainland China and Hong Kong. The risk of infection is considered very low and it has only caused mild disease.