Influenza viruses can mutate
Influenza viruses that infect animal species can mutate and infect humans. The human immune system may have no defences against viruses that previously only infected animals. That's why infection with these viruses can result in more severe disease in people.
If the H5N1 bird flu virus were to mix with a human influenza virus, such a Combined-virus could create a new human influenza virus that could spread rapidly.
Health experts are concerned that the current bird flu affecting parts of Asia, the Middle East, Africa, and Europe could become a worldwide pandemic if the virus does mutate. The worst influenza pandemic in modern history was the Spanish flu, which occurred in 1918??9 and killed up to 50 million people.
Measures to contain the spread of the current bird flu virus include identifying and culling affected poultry flocks, research into tests and vaccines, and rigorous quarantine practices.