The annual pilgrimage to Mecca called Hajj could be a massive issue if swine influenza poses a threat six months from now, flu experts have revealed.
Specialists questioned at a major medical conference in Helsinki shuddered at the implications if a highly contagious, novel flu virus were unleashed at the world's biggest annual gathering.
In the grimmest scenarios, the pathogen would not only find easy pickings among the elderly, the weak and sick in Mecca, it would also hitch a plane ride among pilgrims returning home and thus spread farther.
"Just imagine, you have a virus that starts to spread over the world, then you bring people together from all over the world, put them all together for a couple of weeks, then you take them out again," said Albert Osterhaus, a professor at the Erasmus Medical Centre at the University of Rotterdam in the Netherlands.
"If there's a mechanism by which you want to spread a virus, this is it."
Last December, an estimated two and a half million worshippers travelled to Islam's holiest site for the three-day fulfilment of their faith. This year's pilgrimage takes place November 25-28.
In interviews with AFP at the European Congress of Clinical Microbiology and Infectious Diseases (ECCMID), Osterhaus and other specialists cautioned against alarmism, but urged Hajj organisers to start formulating response plans.
"The Hajj will take place, it's not like one of those things which is like a Pink Floyd concert, and you say, 'we don't need the concert'. This event will go through, that's for sure," said Andreas Voess, professor of Radboud University Nijmegen Medical Centre in the Netherlands.
"I believe that the health authorities there, with the help of the WHO and others, would need to look at what do we do, what do we advise people... they have to be prepared and they have to start thinking about what to do now and not when they have got the first pilgrimage victim with influenza."
He added: "It is something that has to be looked at, it really does."
Since influenza (A)H1N1 swine flu leapt into the spotlight on April 24, nearly 8,500 people have fallen sick, according to the UN's World Health Organisation (WHO).
After originating in Mexico, it has swept into at least 39 countries and the WHO describes a pandemic as "imminent."
At present, the viral strain is considered relatively mild.
Even though it is a new genetic mix to which people do not appear to have immunity, it is roughly as contagious and virulent as normal (also called "seasonal") flu, which breaks out every year in slightly different strains and kills between quarter of a million and half a million people annually.
What will happen to the new virus in the coming months is the big unknown, creating a dilemma for watchdogs hoping to protect the Hajj.
"We cannot predict what will happen by that time, it might be striking, it might not, it might fade away," Osterhaus said.
One of Europe's top virologists, Osterhaus pointed to three ways the virus could go.
It could be ousted in the battle for Darwinian supremacy by the seasonal virus.
Or it could spread, developing into a pandemic that, by the standards of past killer flus, would be low-level. He drew a parallel with a 1957-58 pandemic that killed between one and four million people.
For pilgrims, the question is whether a pandemic vaccine will be available in time, and in sufficient quantities, to innoculate them.
The third, most frightening, scenario is that H1N1 could pick up genes by reassorting with other flu viruses, making it both more lethal as well as highly contagious. The nightmare benchmark for this is another H1N1 strain that in 1918-19 killed around 50 million people through "Spanish flu."
Pentti Huovinen, a professor of clinical microbiology at Finland's National Public Health Institute, said the challenge was to balance "the importance of religion and the threat of the disease."
"That's a question that we cannot answer very easily," he said.