Every year, the single largest gathering on the planet is the annual pilgrimage to Mecca: 2.5 million people from 160 countries packed into a small city in Saudi Arabia for five days.
This year, some will be bringing swine flu.
The Saudi authorities, fearing that the hajj could turn their holy city into a petri dish for viral mutations and a hub for spreading a new pandemic wave around the world, are working hard to head that off. They have asked some worshipers, including pregnant women and the elderly, not to make the trip, which is scheduled for the last week of November.
"The hajj is a central ritual of Islam, and our country tries to make it easy for everyone to come," said Dr. Ziad A. Memish, the country's assistant deputy minister for preventive medicine. "We've said we won't turn away anyone who arrives at our borders. But we are recommending to other countries whom they should let come."
Although the Saudis have turned to the World Health Organization and other health agencies for help in previous public health threats to the hajj, this year the Centers for Disease Control and Prevention, the American government's lead disease-fighting agency, is more deeply involved because it has so much experience with this new flu strain. Consultants for the centers have gone back and forth to Riyadh, flu experts at American medical schools have been called in and the United States Navy's medical laboratory in Cairo is preparing to help with any complex flu testing that is beyond what Saudi laboratories can do.
While religious pilgrimages feed the souls of those who attend, they often endanger the bodies. There have been several outbreaks of meningitis in Mecca since 1987, and in 2004, Muslim pilgrims spread polio from northern Nigeria across Africa to Saudi Arabia and from there outward to Yemen and Indonesia.
In July 2008, about 200,000 Catholics from all over the world converged on Sydney, Australia, for World Youth Day, attended by the new pope, Benedict XVI, during the Southern Hemisphere's winter. There was a major flu outbreak, and a Tamiflu-resistant strain of seasonal flu established itself and then spread to the Northern Hemisphere, including to the United States, last winter.
The Saudis reacted because this new strain is the first pandemic flu since 1968. Any new flu carries the risk of gene-swapping that can form mutant viruses, and this one has some swine and avian genes that, before this April, had never been seen in humans. Both the new strain and seasonal flus will be circulating in the world, increasing the risk of flus mixing in Mecca. Also, although the flu infects younger people, the ones most likely to need hospitalization or die if they do get infected are the very young, pregnant women, the sick and the aged.
The hajj offers many opportunities to a virus that spreads through the air and lingers on surfaces: Pilgrims crowded into planes, boats, buses and tent cities; the endless ranks of the faithful praying shoulder to shoulder and touching their hands to the floors around the Kaaba, to handrails as they run between the hills Safa and Marwah, or to cups of water from the Zamzam Well.
A paper published online on Thursday in the journal Science describes many of the obstacles to fighting transmission there. Dr. Memish, the Saudi official, is a co-author with several Centers for Disease Control experts.
For example, pilgrims are advised to wash their hands frequently and bring their own surgical masks and hand sanitizers. The first is easy: Islamic law requires washing the hands and face five times a day before prayers. But male pilgrims may not wear anything with stitches in it; they wrap themselves in two lengths of unhemmed white cloth. And women on the hajj -- even those who wear chadors at home -- are forbidden to cover their faces. Also, since Islam forbids alcohol, some believers reject most hand sanitizers.
Senior religious leaders have issued fatwas declaring that masks and sanitizers "are not a problem," Dr. Memish said, but every religion has some conservatives who believe in keeping up standards.
The Saudi government has made many preparations, like buying stockpiles of generic Tamiflu from Cipla, an Indian company. The country has 76 health facilities staffed for the hajj, and intensive-care units have been expanded. For pilgrims, all medical care for problems they develop during their visit is free.
"Saudi Arabia's reputation rests on how the hajj is conducted," Dr. Shahul H. Ebrahim, a Centers for Disease Control epidemiologist who advises the Saudi government, said in a recorded interview with the Science article. "It's a rich country, they take a lot of national pride in it and the king is responsible. I don't think anything is lacking here except vaccines."
It will be impossible to stop the flu from arriving, the authorities acknowledge, and hard to slow its spread. But they are trying to lessen the damage by keeping the most vulnerable pilgrims away. Every country gets an annual limit as to how many pilgrims it can send. This year the Saudis suggested barring anyone who is pregnant, under age 12, over age 65 or suffering from diabetes, chronic lung, heart, liver or nerve disease, or some other conditions. Psychologically, the ban is hardest on the old and the sick, Dr. Memish said.
In countries with large Muslim populations, many applicants get one or two chances in a lifetime at the pilgrimage that every Muslim is supposed to make, and "some people save money for their whole lives to do it," he said. More than half of all pilgrims are over 50. In a normal year, many of those desperate to come before they die are pushed in wheelchairs or carried around the Kaaba and through the other rituals.
The new swine flu vaccine could end all the Saudi worries, but only a few countries have even small amounts available, and the Saudis want anyone with access to it to have the shots at least two weeks before arrival.
China has said it will vaccinate all the 12,700 pilgrims planning to make this year's hajj from China. The United States, from which about 15,000 Muslims go to Mecca each year, has not made them a separate vaccine-priority group.
This article originally appeared in The New York Times .