A new influenza pandemic is being seen by experts as the most significant global public health emergency caused by a naturally occurring pathogen.
Since 1918 when more than 40 million people died in a global flu outbreak there have been two other pandemic outbreaks, 1957 in South East Asia which resulted in 2 million deaths and a further one million died in the 1968 pandemic.
In the last 12 months, the world has moved closer to an influenza pandemic than it has been at any time since then. The crucial factors which might spark off a pandemic are a new virus subtype transmittable to humans, which has the ability to replicate in humans and cause disease, and be efficiently transmitted from human to human.
Health experts are concerned that the current avian influenza virus, H5N1, meets these pre-requisites. The first instance of probable human-to-human transmission was reported in September 2004 in Thailand and even though this appears to be an isolated case, experts are dubious and think others may not have been identified and reported.
Of the 88 identified human cases in Asia, by mid April 2005, 51 of them had died. The main victims have been young, healthy adults and this is puzzling the scientific experts.
The H5N1 virus is now endemic in many south-Asian countries and experts are now aiming at containment rather than eradication. It was first identified in 1997, and since 2004 the avian flu virus has become more pathogenic, surviving several days longer, has been found in mammals and can be asymptomatic - 1/3 of domestic ducks in Thailand are reportedly infected without developing any obvious signs.
An expert from the World Health Organization (WHO) has estimated that even in the best case scenario the global death toll could be between 2 and 7 million people.
The WHO were urging all governments in January this year to draw up and implement a national preparedness plan and to strengthen global influenza surveillance.