Through the eyes of the H1N1 virus, a Catholic church is a playground. The font of holy water near the church entrance is a great place for the virus to leap from one person to another.
The passing of the peace, during which parishioners shake hands, is yet another favorite place for the virus.
And then there's Communion: The priest puts the host, or wafer, on a parishioner's tongue or into the person's hand, and then does the same for the next person. Often, he then serves wine from a common cup. It's wiped clean each time, but that's no guarantee it's virus-free.
Bishop Mitchell Rozanski of Baltimore said these rituals have prompted a flood of questions.
"How should we deal with the distribution of Holy Communion?" he said. "Should we stop shaking hands at the sign of peace? Should we take [out] the holy water fonts as soon as the flu season begins?"
Rozanski is asking priests to use lots of hand sanitizer. But the bishop said a word from health officials that a pandemic has started could lead him to shut down churches.
And what about mosques?
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For Muslims, Friday prayers are the centerpiece of the faith and a potential viral hotbed. Imam Johari Abdul-Malik of Dar Al Hijrah Islamic Center in Falls Church, Va., says 3,000 people come to worship there each week.
"They stand shoulder to shoulder. They put their faces on the carpet and remember someone's going to come behind them and prostrate with their face on the carpet," he says.
Malik says that if flu breaks out, the leaders may ask people to bring in their own prayer rugs. The mosque may tell people to spread out more when they pray and amend the ritual cleansing before prayer, in which people visit a special washroom to gargle and wash their feet.
The imam said believers are loath to change these rituals because they're so deeply rooted in the Quran. "How do you now convince them that what they used to do is now not permissible?" he asked.
For example, Malik says, the Quran says that when two believers shake hands in greeting, their sins fall away. Some people may feel cheated of blessing if they have to stop that practice.
"You come to the mosque, and nobody wants to shake your hand, no one's going to embrace you. You're like, 'What's going on? What's kind of place is this? Where are the blessings?' " he said. "And you have to say, 'Today the blessing is in resisting shaking hands.' "