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Bird flu was first identified in the early 1900's and has since spread worldwide. Also known as avian influenza, this virus has caused considerable concern due to the mutation of a particular strain of the disease. Although this virus previously only infected birds and other types of animals, namely pigs, since 1997, it has also been known to infect humans.

The strain of the disease to cause so much concern is H5N1. These are simply numbers and letters that represent the subtype of this particular strain, 1 of 144 influenza subtypes. Not only has the virus caused an epidemic in poultry, but it has recently been feared to be leading to a pandemic, or worldwide epidemic, in humans.

While the virus was first identified in humans in 1997, it was not until 2004 that the spread became of great concern. At that time, a major outbreak occurred in Vietnam and Thailand, which spread to ten countries and regions of Asia within weeks and caused the death of 23 people. Within three months the outbreak was contained after the slaughter of tens of millions of potentially infected birds. However, the damage was already done and the virus had spread across Asia to lead to additional outbreaks. Since that time, H5N1 has spread throughout Asia, Europe, and the Middle East, and a low pathogenic form of the virus was identified in Canada on November 19, 2005. Currently, 131 humans have been infected with the virus, resulting in 68 deaths. However, it is feared this number will only increase with the ongoing spread of the disease.

The primary concern surrounding H5N1 is its mutation and ability to infect humans. As of yet, the virus has been spread from poultry to humans, and human to human transmission has only been suspected but not confirmed. Once the virus mutates further, it will easily be passed through humans, causing the disease to spread rapidly. Influenza pandemics, or worldwide epidemics, have caused a great number of deaths in the past, including the Spanish Flu which killed 50 million people in 1918. This is the ultimate concern with the mutation and spread of H5N1.

At this time, the primary cause of infection has been due to the consumption or handling of diseased poultry. Unfortunately, there have been a very few cases that were not easily explained, and therefore, human to human transmission was suspected. However, this has not been confirmed in any of the cases of H5N1 infection.