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Historically, the deadliest influenza pandemics begin as viruses that just infect animals, like birds or swine, and progressively build to a point where they infect and be transmitted between humans. At this time, all we really know is that the risk of bird flu turning into an influenza pandemic is significant, at least as great as it has been since 1968, when the last flu pandemic emerged. That is why world leaders have been discussing the risk of flu pandemics and stockpiling tens of millions of doses of flu vaccine. Very often, pandemic flu viruses have both bird flu genes and human flu genes. An alternative name for bird flu is avian flu.
According to the World Health Organization (WHO), there are six levels of alert for flu pandemics:
1. Level 1, code green, low risk of human cases. Inter-pandemic phase.
2. Level 2, code green, higher risk of human cases. Inter-pandemic phase.
3. Level 3, code yellow, pandemic alert, no or very limited human-to-human transmission.
4. Level 4, code yellow, pandemic alert, evidence of increased human-to-human transmission.
5. Level 5, code orange, pandemic alert, evidence of significant human-to-human transmission.
6. Level 6, code red, pandemic in progress, efficient and sustained human-to-human transmission.
According to the WHO, we are currently at Level 3. Several dozen people have been infected by bird flu in the last couple decades, but the virus could not be transmitted between humans. The main victims are people working very closely with animals in Third World countries under poor hygienic conditions.
Based on historical data, the entire cycle completes itself every 20 to 100 years. There were three flu pandemics in the 20th century, and none so far in the 21st.
There are two means by which bird flu or swine flu may be mutated into a pandemic flu virus. One is simple mutations leading to a version of the virus that can infect and spread between humans. Another is a human being infected by the bird flu and the human flu simultaneously, and the viruses exchanging genetic material in a process called association, leading to a new virus that can be transmitted between humans.
Influenza pandemics are the deadliest events in human history. The 1918 flu pandemic killed between 50 and 100 million previously healthy people, greater than the toll from World War I, which had occurred just prior to the outbreak.