Outbreaks of bird flu in humans have been rumbling for some years but usually only cause milder problems such as conjunctivitis and are rarely fatal.
As 2004 wound dramatically into 2005 the deaths of two young boys in Vietnam were overshadowed by the disastrous tsunami that swept across the Indian Ocean. But the boys' deaths, from a strain of influenza known as influenza A H5N1- or more simply 'bird flu' - could herald a far greater loss of life. In 1918, a pandemic of flu swept the world killing more than 20 million people, and many infectious disease experts now believe that another flu pandemic could be imminent. The appearance in humans of a type of flu which normally only affects birds may be an important sign that some strains of the virus are changing in a way that could threaten people around the globe.
Avian flu is becoming more dangerous
Influenza A viruses naturally occur in wild birds. Although these birds aren't affected by the virus, domestic poultry such as chickens and turkeys are - and so are people.
However, the strain found in the Vietnamese boys seems to be more dangerous. More than 90% of birds who get H5N1 die, and mortality among humans is also high. The H5N1 virus was first shown to have passed from birds to humans in 1997, during an outbreak of avian influenza among poultry in Hong Kong. The virus caused severe respiratory illness in 18 people, of whom six died. In the past year or two there have been a stream of cases reported by the World Health Organization (WHO) in Asia. Since January 2004, there have been 47 confirmed cases of H5N1 and 34 deaths in Vietnam and Thailand. More worryingly, recent research has shown that H5N1 has changed so that it's even more deadly in chickens and mice, and can now infect cats too. H5N1 is also resistant to some of the drugs used to treat flu (such as amantadine).
H5N1 has become common among birds in Asia, who shed the virus in their saliva, nasal secretions and faeces. Millions of chickens and ducks have been slaughtered across South East Asia in recent months in an effort to prevent spread of the virus from birds to humans. Work is also underway to make a vaccine against H5N1. But it will take time and that is in short supply. If H5N1 becomes able to pass from human to human then the situation will be even more serious as most people have little immunity to the strain and there will be rapid spread. In September 2004, the first possible case of human-to-human transmission was reported in Thailand.
Flu viruses continually change
One of the most worrying features of the viruses which cause flu is the way that they are continually changing over time, through small changes in their make-up called antigenic drift and occasional abrupt major changes called antigenic shift.
This can mean that, although you might have fought and won a miserable battle against flu a year or two ago, the next time the virus appears your body won't recognize it. The antibodies your body made against flu last time won't work. So the immune system must learn to fight it all over again. This is why people need to be immunized against flu each year, using the most up-to-date strains of the virus.
There are three types of influenza viruses: A, B, and C. Influenza C only causes mild problems in humans. Influenza B can cause more serious illness and seasonal epidemics, but because it only changes through the slower process of antigenic drift there is little risk of a pandemic, where millions of people are suddenly exposed to a new, dramatically different virus. That threat is more likely to come from Influenza type A which does undergo shift.
New strains of flu tend to emerge in Asia and the East, where people live in closer quarters with their animals and different flu viruses may mix to cause new strains, or pass between species. For example, domestic ducks may carry H5N1 without symptoms and release the virus for long periods, acting as what is known as a silent reservoir. In rural areas free-ranging ducks and chickens often mingle and share the same water supplies. The virus may then be passed from one animal to another and possibly even to humans.