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.