Bird flu poses no immediate threat to the average person. The only people who have been infected with the highly pathogenic H5N1 bird flu that has periodically caused epidemics in poultry in the last decade are those who have come into close contact with sick poultry.
There has never been an H5N1 outbreak in poultry in the United States, although a highly pathogenic H5N2 strain caused an outbreak in 1983-4 that was contained by destroying 17 million birds. No people were infected. Global surveillance is in place to detect and contain outbreaks in birds, and researchers are working to understand whether and how such strains could adapt to humans.
An H5N1 strain that first emerged in domestic chickens and ducks in 1997 caused huge outbreaks with very high mortality in poultry flocks around the world over the past decade. It is also known to have infected at least several hundred people. In these cases, the H5N1 virus infected cells deep within the lung (where the temperature is several degrees warmer than in the nose), and spread very poorly, if at all, from human to human. However, when it did manage to establish an infection deep in the lung, it caused a severe pneumonia that was often fatal.
Public health officials are concerned that if the H5N1 virus were to adapt so that it could spread more efficiently, it might cause an extensive pandemic with very high mortality. There are a great many unknowns about the adaptation process. For example, if the virus adapts so that it grows efficiently in the upper respiratory tract, it might no longer cause such severe disease.
Nevertheless, since no influenza strain with hemagglutinin #5 has ever circulated in humans, no one would have any immunity and the pandemic could be very extensive. As a result, a global surveillance effort has been undertaken to ensure that outbreaks in poultry are contained promptly. Researchers are working to understand the process of adaptation in the hope of detecting early changes in the virus that might foretell an imminent jump to humans. Furthermore, H5N1 vaccine seed strains have been developed so that a vaccine could be developed quickly should the virus develop the capacity to spread from person to person.
What about H7N9?
In March 2013, several people were found to be infected by an avian-adapted strain of the subtype H7N9. As of April 17th, 82 cases had been verified, with 17 deaths. There was at that point no evidence that the virus could spread from human to human. An unusual feature of the H7N9 strain is that unlike the highly pathogenic H5N1, it does not cause severe disease in poultry, raising concerns that its presence in poultry flocks and live bird markets could take longer to detect.
Source: This excerpt is taken from FLU: FAQs of The American Academy of Microbiology. The authorship is this article rests on AAM and excerpt is published here with permission from AAM.