Throughout time, the flu virus has changed considerably and evolved into various "versions" of itself. For example, the virus held responsible for the Spanish Flu in 1918 (the deadliest flu epidemic of the modern era) was the H1N1. In 1957, another version of the flu virus, the H2N2 caused the Asian Flu, killing over 1.5 million people. A decade later, in 1968, the Hong Kong flu was triggered by the H3N2 virus, so as you can see, the virus finds new forms to suit its spreading and infection and to resist the internal and external factors that could hinder its spread. Recently, in 1997, a new strain of the virus was discovered, the H5N1 which mostly affects birds at the moment, although some cases of human infection have been reported. The new disease was called the "bird flu" because it spreads rampantly among birds.
The bird flu, or avian flu (avian influenza) as it is sometimes referred to is part of the Influenza A virus family. Mistakenly, some people consider the "A" in the family's name as coming from "Avian", however it is not the case. It is just a classification of the virus subtypes inside the Orthomyxoviridae family.
Before the H5N1 virus was discovered, the avian flu was not treated with considerable interest by the medical and scientific community, since it would only affect birds at a lower rate and even if humans did become infected, the virus would be inactive or would produce minor, insignificant effects. H5N1 however, brought the danger of the bird flu to a whole new level. The H5N1 type of bird flu is highly dangerous to humans, although it is still not yet easy to contract. It would take a rather strange twist of events for the flu virus to infect the human body straight from a bird, since the virus can only transmit itself through the bird's mucus, saliva or feces. If the virus does reach the human body, it may have deadly effects, especially through SARS, or Severe Acute Respiratory Syndrome.
Although there have only been a small amount of documented cases in which the virus took a "species leap" and infected a human, the fact that this DID happen triggered an alarm worldwide. Although the bird flu virus is not yet powerful enough to cause a pandemic among humans (it has the capability of starting a pandemic among poultry still), the situation should not be treated lightly as the virus could easily evolve into a strain that would be easier to transmit and deadlier in effect.
Compared to other types of flu, the avian influenza does not have symptoms that are radically different, however they are a lot quicker in effect and more powerful. So for example, the fever caused by a "normal" flu is less dangerous than the fever caused by the bird flu. In addition, bird flu will quickly evolve into other, more dangerous complications such as the Severe Acute Respiratory Syndrome, or a form of pneumonia that is extremely resistant to antibiotics and other medications.
