The virus can change its antigenic structure suddenly and repeatedly. Due to this it can escape the immune response of the body that has been gained by influenza vaccine or recent infection. This property makes them more dangerous. There are three types of influenza viruses A, B and C.
Type A influenza virus causes serious flu pandemic. Type B influenza virus causes localized and smaller outbreaks. Type C influenza virus causes milder disease and the strains of the virus are more stable and the disease due to this type is less common.
Type A and B cause the disease more or less in every winter season. Type A virus can cause disease in humans and in animals. It can infect whales, seals, horses, pigs and birds. Type B and C are generally found only in humans.
The virus strains which can infect various hosts originate in the areas where people live in close contact with pigs and chickens. Pigs are susceptible to viruses from both birds and humans. The transfer of genes from different viruses takes place in pigs and new strains are created. At least 15 flu subtypes affect birds, the most virulent of which is H5N1.
How humans get avian flu
Avian viruses generally do not affect humans, but in 1997, an outbreak of bird flu in Hong Kong infected 18 people, six of whom died. Since then, human cases of bird flu have been reported in Asia, Europe and the Middle East. Most were traced to contact with infected poultry or surfaces contaminated by sick birds.
Often, flu viruses that cross the species barrier originate in areas where people live in close proximity to chickens and pigs. That's because pigs are susceptible to infection with both avian and human viruses and so are an ideal "mixing bowl" for genes.
But at least some bird flu viruses don't need a third party. Instead, they shuffle and rearrange their genetic material directly in humans. That seems to be the case in most instances of human-acquired bird flu. People become sick after direct contact with infected birds or bird-contaminated surfaces, not from contact with other animals.
Direct bird-to-human transmission works like this:
Infected migratory waterfowl, the natural carriers of bird flu viruses, shed the virus in their droppings, saliva and nasal secretions.
Domestic poultry become infected from contact with these birds or with contaminated water, feed or soil. Bird flu spreads quickly within a domestic flock and is inadvertently transported from farm to farm on equipment, cages, and workers' shoes and clothing. Heat destroys the virus, but it can survive for extended periods in cool temperatures.
Open-air markets, where eggs and birds are sold in crowded and unsanitary conditions, are hotbeds of infection and spread the disease into the wider community. At any point along the way, humans may pick up the virus through close contact with sick birds or contaminated surfaces. An ailing bird can shed the virus in its feathers as well as in droppings, and some people have contracted bird flu simply by touching an infected bird. Young children are found to be more susceptible to infection. It may be because play on the ground where the droppings of sick birds are mixed with the soil.
The ease of worldwide travel has the potential to spread bird flu around the globe. And migratory birds can carry the virus from continent to continent along flyways. Outbreaks may also spread locally through unsanitary markets, contaminated clothing and equipment, and smuggled birds.
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Symptoms of avian flu are similar to the symptoms of common influenza. The symptoms are cough, fever, muscle pain and sore throat. Sometimes the only manifestation is a mild infection of the eye akin to conjunctivitis.
Tests & tools
Laboratory tests can identify flu virus but tests to identify influenza from avian flu are under research.
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