What, why, when ...?
Bird flu is also called avian influenza. Fifteen strains of flu affect birds, but the one causing the global scare is the H5N1 subtype. It has circulated in migrating wild birds for years and has spread to poultry flocks, starting in South East Asia, spreading to Russia and now reaching Turkey. It is highly lethal to domesticated birds.
How it spreads?
Bird flu was first identified in Italy around a century ago. It was not thought to be transmissible to humans until 1997, when the first human cases were seen in Hong Kong, involving H5N1.
In the latest outbreak, around 60 people in Asia have died, amounting to roughly half the known number of infections, which is a very high fatality rate. Almost all these casualties were exposed to infected fowl, making contact with the virus through the birds' saliva, nasal secretions and faeces, which become dry, pulverised and are then inhaled. Avian flu is not a food-borne virus, so the risk from eating properly-cooked chicken is negligible.
Symptoms
Bird flu in humans causes symptoms that are like human flu, such as fever, cough, sore throat and muscle aches, conjunctivitis, pneumonia and other severe respiratory diseases.
Risk factors
At present, H5N1 is not easily transmitted from bird to human. In other words, a person would have to pick up a lot of virus in order to be infected. Nor is it easily passed from human to human: there have been only three suspected cases, in Thailand, Hong Kong and Vietnam, where this is believed to have happened. The big worry, though, is that H5N1 could pick up genes from conventional human flu viruses, making it highly lethal and highly infectious. Because it would be a radically new pathogen, no one would have any immunity to it.The mutation could occur if H5N1 co-infects a human who already has ordinary flu or the agent is picked up from poultry by an animal that can carry bird and regular flu strains.
Past pandemics
In the 20th century, there have been three flu pandemics, in 1918-19, 1957-58 and 1968-69. The worst of all was the 1918-19 pandemic, which killed as many as 50 million people - more than the death toll from Aids in more than two decades.
Villagers look at a health worker as he culls chickens at a poultry farm in Margram village, about 240 km (149 miles) north of the eastern Indian city of Kolkata January 17, 2008. Villagers at the centre of a bird flu outbreak in India's east refused to hand over their chickens and ducks for culling on Thursday, hampering efforts to stamp out the deadly disease in poultry. REUTERS/ Parth Sanyal (INDIA)REF: RTR1VX3F
Health workers burn their bird flu protective gears after culling chickens outside a poultry farm in Margram village, about 240 km (149 miles) north of the eastern Indian city of Kolkata January 17, 2008. Villagers at the centre of a bird flu outbreak in India's east refused to hand over their chickens and ducks for culling on Thursday, hampering efforts to stamp out the deadly disease in poultry. REUTERS/Parth Sanyal (INDIA)REF: RTR1VX18
A vendor transports chickens on a bicycle from a wholesale market in the eastern Indian city of Kolkata January 17,2008. Villagers at the centre of a bird flu outbreak in West Bengal refused to hand over their chickens and ducks for culling on Thursday, hampering efforts to stamp out the deadly disease in poultry. REUTERS/Jayanta Shaw (INDIA)REF: RTR1VWVQ
Villagers stand near health workers preparing to cull poultry at Margram village, about 240 km (149 miles) north of the eastern Indian city of Kolkata January 16, 2008. Veterinary workers began killing thousands of chickens as fears mounted that bird flu could have spread to a third district in West Bengal. REUTERS/Parth Sanyal (INDIA)REF: RTR1VVUV
Vendors display chickens at a wholesale chicken market in the eastern Indian city of Kolkata, January 16, 2008. Veterinary workers began killing thousands of chickens as fears mounted that bird flu could have spread to a third district in West Bengal. Officials said it could take up to a week to cull about 400,000 chickens in two districts of the state where the latest outbreak of H5N1 avian influenza has been detected. REUTERS/Jayanta Shaw (INDIA) REF: RTR1VVOQ
A health worker stands near villagers as he waits to begin culling chickens at Bamdevpur village, about 250 km (155 miles) north of the eastern Indian city of Kolkata January 16, 2008. Veterinary workers began killing thousands of chickens as fears mounted that bird flu could have spread to a third district in West Bengal. Officials said it could take up to a week to cull about 400,000 chickens in two districts of the state where the latest outbreak of H5N1 avian influenza has been detected. REUTERS/ Parth Sanyal (INDIA)REF: RTR1VV84
Ducks are pictured inside a net at a wholesale market in the eastern Indian city of Kolkata, January 16, 2008. Veterinary workers began killing thousands of chickens as fears mounted that bird flu could have spread to a third district in West Bengal. Officials said it could take up to a week to cull about 400,000 chickens in two districts of the state where the latest outbreak of H5N1 avian influenza has been detected. REUTERS/Jayanta Shaw (INDIA)
REF: RTR1VVN1
REF: RTR1VVN1
A villager brings bird stock to health workers for culling at Bamdevpur village, about 250 km (155 miles) north of the eastern Indian city of Kolkata January 16, 2008. Veterinary workers began killing thousands of chickens as fears mounted that bird flu could have spread to a third district in West Bengal. Officials said it could take up to a week to cull about 400,000 chickens in two districts of the state where the latest outbreak of H5N1 avian influenza has been detected. REUTERS/Parth Sanyal ( INDIA)
REF: RTR1VV7Y
REF: RTR1VV7Y
A villager brings bird stock to health workers for culling at Bamdevpur village, about 250 km (155 miles) north of the eastern Indian city of Kolkata January 16, 2008. Veterinary workers began killing thousands of chickens as fears mounted that bird flu could have spread to a third district in West Bengal. Officials said it could take up to a week to cull about 400,000 chickens in two districts of the state where the latest outbreak of H5N1 avian influenza has been detected. REUTERS/Parth Sanyal ( INDIA)REF: RTR1VV7T
A poultry owner buries his dead chickens in a paddy field in Margram village, about 240 km (149 miles) north of the eastern Indian city of Kolkata, January 14, 2008. India began culling thousands of chickens on Monday and checking people for fever in a remote eastern village after preliminary tests on dead poultry showed they were infected with bird flu.REUTERS/ Stringer (INDIA)REF: RTR1VSOD
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