The H5N1 virus does not usually infect people, but more than 130 human cases have been reported by the World Health Organization (WHO) since January 2004. Most of these cases have occurred as a result of people having direct or close contact with infected poultry or contaminated surfaces. However, a few cases of human-to-human spread of H5N1 have occurred.
Of the few avian influenza viruses that have crossed the species barrier to infect humans, the H5N1 virus has caused the largest number of detected cases of severe disease and death in humans. In the current outbreaks in Asia and Europe, more than half of those infected with the virus have died. Most cases have occurred in previously healthy children and young adults. However, it is possible that the only cases currently being reported are those in the most severely ill people and that the full range of illness caused by the H5N1 virus has not yet been defined.
So far, the spread of H5N1 from person to person has been rare and has not continued beyond one person. Nonetheless, because all influenza viruses have the ability to change, scientists are concerned that the H5N1 virus could, one day, be able to infect humans and spread easily from one person to another.
Because these viruses do not commonly infect humans, there is little or no immune protection against them in the human population. If the H5N1 virus were to gain the capacity to spread easily from person to person, an influenza pandemic (worldwide outbreak of disease) could begin. Although no one can predict when a pandemic might occur, experts from around the world are watching the H5N1 situation in Asia and Europe closely, and are preparing for the possibility that the virus may begin to spread more easily and widely from person to person.