looking for this sf novel about an ex soldier who during a pandemic is charging rich parents space for their kids in a secure monastery to survive

I'm a librarian and sf lover who's been asked to help locate a book which sounds very familiar to me, but which I never read myself. I haven't had any luck searching our catalogue or various sf topical indexes, so am hoping someone out there has read the book. We're looking for the author and title.

Here's my patron's description: "[It's] about an ex soldier who during a pandemic is charging the rich parents for their kids [to have a space in] a secure monastery to survive. As it turns out the monastery is not so secure, one of the kids stumbles across an old monk (??) who has survived in the catacombs of the place. Turns out he is a carrier or the virus has survived in the dead bodies in the tombs, I forget which. ... I believe you are right in that the soldier had advance knowledge of the virus. I think the opening took place in California but I'm not 100% sure. As far as the monastery goes I don't think it really specified but the description reminded me of the Greek monastery Meteora on the cliffs. ... I believe the virus was a flu virus in nature. But it caused massive hemorrhaging and extremely high fever. The soldier had a female assistant nurse who became very close with the children."

He read the book about five years ago, and my own hazy memory of having seen the book jibes with that; I think it must have been first published sometime between 1999 and 2010. Thanks so much!