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There are hundreds of different strains of avian influenza A virus, all of which can cause bird flu to varying degrees in birds. However, influenza viruses as a family are very species specific, rarely capable of transference between species. Only four strains of avian influenza A virus have been found in humans since 1959: H5N1, H7N3, H7N7 and H9N2. The last three produce only mild flu symptoms in people; the viral strain arousing significant concern the world over is H5N1, which the World Health Organization reports has caused 254 human deaths up to February 5, 2009. It is highly contagious between birds and frequently deadly for both domestic and wild bird species. Domestic flocks raised under poor welfare conditions are particularly susceptible, as their high stress situation inhibits their immune system performance.

The H5N1 bird flu affects different people in different ways; it is also liable to mutation so the symptoms may change. However, the majority of people infected to date have displayed the following symptoms:

Fever, over 38 degrees Celsius (100.4 degrees Fahrenheit).
Headache, can be severe.
Sore throat.
Debilitating fatigue.
Watery and bloodless diarrhea.
Chest pain.
Abdominal pain.
Vomiting.
Bleeding from the nose and gums.
Difficulty breathing from 5 days after first symptom.
Varying amounts of sputum from the lungs, that may be bloody.
Untreated, it will lead to viral pneumonia.

Some of those infected have failed to display any of the respiratory symptoms. Two cases in southern Vietnam were initially diagnosed as acute encephalitis because of this. Another patient in Thailand only had the fever and diarrhea when they first presented. The fever is the one constant, symptomatic in all cases so far.

This is a particularly virulent disease that follows an aggressive and relentless course once symptoms develop. Deterioration in the patients health is rapid, and death has resulted in 64% of cases so far. The incubation period, which is the time between infection and the appearance of symptoms, is not completely clear as yet. The first cases in Hong Kong in 1997 ranged from two to four days, but later cases give a larger spread from two to eight days with the possibility the incubation period in rare cases may be up to 17 days.

The occurrence of bird flu in humans is most prominent in those who handle domestic birds, particularly live birds or those plucking feathers from bird carcases. Influenza viruses are predominantly spread via

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