URBANA, Ill. - Mold has infested the University of Illinois' rare book library, and officials said the estimated $800,000 worth of cleanup will force it to close for months.

About 15,000 books have visible mold and more damage may be discovered, university officials said.

Librarian Tom Teper said the 300,000-item collection in the Rare Book & Manuscript Library is worth more than $1 billion. It includes papers and letters of Carl Sandburg, H.G. Wells and Marcel Proust and first editions of titles by Charles Dickens and Mark Twain.

Exactly which pieces are harmed hasn't been announced, but Teper said Friday that the mold problem "is scattered throughout the collection." He said it appears everything can be restored and there is no health risk.

"We are really looking to protect this collection that is irreplaceable and really a gem in the state of Illinois," Teper said. "People come from all over to look at this material."

The collection, housed in a vault within the school's main library on the Urbana-Champaign campus, has a ventilation system that is more than 25 years old. Controlling temperature and humidity is challenging, Teper said.

Officials said they'll shut down the library Feb. 25 so the books, shelves and ventilation system can be cleaned. That cleanup will be done by an independent company, Texas-based BMS Catastrophe. Cleaning the rare books will be accomplished by a special vacuum with a filter fine enough to trap mold spores, Teper said.

Teper said the collection is insured. The university administration has supported the cleanup efforts, he said.

The rare book library is slated to reopen May 5.