WHO has declared the swine flu spread a 'pandemic'. But is that announcement cause enough for panic?
For many, the term may be tinged with fear. It evokes folk memories of three influenza pandemics that erupted last century and claimed tens of millions of lives.
The worst was the 1918-19 "Spanish flu."
The greatest plague of the 20th century killed as many as 50 million people, particularly the young and healthy, who could be dispatched to their grave in just a few days, their ravaged lungs filled with blood.
But health experts are keen to defuse any "we're all going to die" reflex after the World Health Organisation (WHO) on Thursday announced a flu pandemic was underway.
For one thing, "pandemic" is only a technical term that indicates the geographical spread of a disease.
Despite its scary connotations, the word is no indication as to how contagious or lethal the disease is.
"You can have serious pandemics, and you can have wimpy pandemics," notes Albert Osterhaus, a well known virologist at the University of Rotterdam in the Netherlands.
Nor does the term apply only to influenza: the world already has pandemics of AIDS and malaria.
Together, they kill around three million people a year and infect millions more. They may cause grief and fear, but not panic.
The reason is that these pandemics have been established for decades, which means people deem them quantifiable risks, rather than a new, apparently random and thus terrifying peril.
Adam Kamradt-Scott, research fellow at the London School of Hygiene and Tropical Medicine at the University of London, says keeping a sense of proportion is essential as the world confronts the new flu virus.
"Even in the worst-case scenario, if this turns out to be a particularly nasty strain with around 25 percent of the population affected, the bulk of people are going to recover and lead normal lives healthwise," Kamradt-Scott told AFP.
"There is a risk that some people could die, but ultimately the majority of people who contract it will recover. So people need to be cautious and take precautions, and act on scientific evidence and not panic."
How lethal the new flu is, or could become, is a question for virologists and epidemiologists -- specialists in analysing a pathogen's genetic ID and how it propagates.
Pandemic viruses are microbes that have acquired new genetic material while mixing in an animal host -- usually a pig, which is able to harbour bird, flu and swine viruses simultaneously -- and then leap the species barrier.
The new genes mean people have no immunity to the virus, as they have not been exposed to it before. And as the virus spreads among humans, the strain is likely to further mutate.
"After emerging into a population it may acquire sudden virulence," explained Patrick Berche, professor of microbiology at the Necker Hospital for Sick Children in Paris.
"Then, when more and more of the population build up immunity to it, the virus starts to lose its virulence."
The pandemics of so-called Asian flu in 1957-58 and of "Hong Kong" flu in 1968-69 killed up to four million people and around a million respectively, according to varying estimates.
Their case fatality rate was around 0.1 percent. By contrast, mortality for "Spanish flu" was 25 times higher -- "The Mother of All Pandemics" is how US virologists Jeffery Taubenberger and David Morens describe it.
By resurrecting the virus, recovered in scraps among frozen corpses in Alaska, and then testing it on lab animals, Taubenberger and colleagues found it had a unique combination of genes that caused the immune system to run amok.
There is a host of factors other than genes that determine the toll from a pandemic.
These include the speed at which it travels geographically, the proximity of people, the season (winter is more favourable to the virus than summer), and, of course, medical preparedness and precautions taken by individuals and governments.
What makes the world more vulnerable in 2009 as compared to 1918 is the advent of jet travel, which means a virus can travel continents in just hours, and a population that has surged from two billion to six billion.
"But the advantages are that we have antivirals and antibiotics," said Berche. "In 1918, many deaths were due to secondary bacterial pneumonia following viral infection."
"In developed countries, we're no longer in 1918," said Joseph Ajjar, an epidemiologist who is head of the French Society of Hospital Hygiene. "On the other hand, I fear the ones who will pay a heavy price are developing countries."