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The only avian influenza virus that has made people sick is the H5N1, but as written in the other threads in this forum, the infections to humans have been very limited and transmission of the H5N1 from one human to another is very weak.

The scare is due to the nature of viruses that can mutate quickly. Viruses that normally do not infect humans can mutate and suddenly infect people. If a new strain of virus starts affecting humans, it can quickly spread from one human to another similar to the 2009 H1N1 virus. The other concern is that a vaccine is not available for a new strain of virus and by the time a vaccine is developed, the virus can take a toll on human lives. Due to the fact that viruses can mutate, they can also become resistant to drugs and the commonly used anti-influenza drugs may not be effective at all in that case.

According to the CDC, influenza A viruses have eight separate gene segments. The segmented genome allows influenza A viruses from different species to mix and create a new influenza A virus if viruses from two different species infect the same person or animal. For example, if a pig were infected with a human influenza A virus and an avian influenza A virus at the same time, the new replicating viruses could mix existing genetic information (reassortment) and produce a new virus that had most of the genes from the human virus, but a hemagglutinin and/or neuraminidase from the avian virus. The resulting new virus might then be able to infect humans and spread from person to person, but it would have surface proteins (hemagglutinin and/or neuraminidase) not previously seen in influenza viruses that infect humans.

This type of major change in the influenza A viruses is known as antigenic shift. Antigenic shift results when a new influenza A subtype to which most people have little or no immune protection infects humans. If this new virus causes illness in people and can be transmitted easily from person to person, an influenza pandemic can occur.