Hajj pilgrims — some wearing surgical masks — walk in Mina, near Mecca, in this photo released by the Saudi Press Agency. (Saudi Press Agency/Associated Press) Millions of Muslims — many wearing surgical masks to ward off swine flu — converged on a holy site in Saudi Arabia to cast pebbles at stone walls representing the devil as part of the hajj pilgrimage.

The annual event in Mina draws three million visitors each year, making it the largest yearly gathering of people in the world and an ideal incubator for the H1N1 influenza virus.

So far, only around 60 cases have been uncovered, but health officials warn it is likely spreading silently among pilgrims and its true extent won't be known until later after the faithful have returned to their home countries around the world.

Saudi officials, along with American and international health experts, have geared up to try to limit any outbreak. But they also are using the pilgrimage as a test case to build a database, watch for mutations and look for lessons on controlling the flu at other large gatherings, such as the 2010 soccer World Cup in South Africa.

In close contact

The stoning of the devil ritual, performed on Friday, Saturday and Sunday, is when the crowds of pilgrims at the five-day hajj are at their height and contact between them is closest.

Under a hot sun Saturday, hundreds of thousands of sweaty bodies pressed against each other toward the stoning walls. The majority did not wear masks, and many sneezed, coughed and spat and looked visibly exhausted.

Other parts of the hajj, such as the circling of the Kaaba shrine in Mecca, see a lot of physical contact and close quarters, but perhaps not as much as the rites at Mina, in a desert valley outside Mecca. The epic crowds squeeze together along ramps and platforms that control traffic around the walls. They push past each other to hurl their pebbles at each wall, often shouting curses at Satan and rejecting his temptations.

"This is when the crowding is at its peak and this is where the spreading is likely to take place," said Hassan el-Bushra, an epidemiologist at the Cairo office of the World Health Organization. "All the hajjis [pilgrims] are in a very limited physical location."

American and Saudi health officials circulated among the sprawling tent camp at Mina where the pilgrims live and gave the faithful cheek swabs for testing later.

Health authorities hung posters of correct handwashing and hand sanitizer dispensers were placed on walls in the camps near public bathrooms and at ritual sites. Pilgrims arriving at Saudi airports were also scanned using a thermal camera and were offered a free vaccine.

So far, four pilgrims have died from swine flu since arriving in Saudi Arabia and 62 others have been diagnosed with the virus after reporting at the hundreds of clinics that have been set up at the holy sites in and around Mecca.

Health officials expressed relief at the low number. But since the flu's incubation period can be as long as a week, the number of cases from the hajj won't be known until after pilgrims return home, starting Sunday. Then it will be up to their home countries to monitor new cases.

"We don't expect there to be a big number of cases in the next two days; it will be the week after hajj when you will see the escalation," said the WHO's el-Bushra.

Many pilgrims already infected may have such mild symptoms they don't even know they are infected. Others simply don't seek treatment.

Saudi Arabia this year recommended that those highly susceptible to H1N1 — the very young and the very old — stay away and that may have had some impact in limiting cases.

Still, some pilgrims were seen carrying newborn babies and dragging infants behind them as they circled the Kaaba. Many elderly pilgrims feebly slumped in wheelchairs as relatives pushed them through the motions of the rites. For the old in particular, it's hard to pass on the hajj, since many want to do it at least once in their lifetimes.

The hajj gives epidemiologists a unique chance to study the virus.

Saudi and CDC experts are working to get cheek swabs from a representative sample of the population for later study. Some pilgrims were swabbed upon entry at the airport and at exit, said Ebrahim. Authorities have also begun taking samples from illegal pilgrims who sneak in by land or sea without hajj visas and have set up sprawling makeshift camps on the rugged mountains of Mina.

The samples will be tested for H1N1 and any new strains of the swine flu that may have developed.

"This data will help with what to do in future mass gatherings, like the World Cup," Ebrahim said.