Not everyone does all their reading on a tablet or smartphone. Some people still crave the feel of a physical book. But if you get your books from the library, have you ever wondered what else you’re carrying home — especially if you’re borrowing a popular book hundreds of hands have touched?

"Seinfeld" fans might remember the episode when George takes an expensive book from a bookstore into the store’s bathroom and is later forced to buy the (presumably contaminated) tome.

So what are you really exposed to when you bring home a library book?

Related: Germ-Proof Your Commute

Mold and mildew

You know that old book smell that makes you sneeze? It’s probably due to mold or mildew. The book may have been stored at some point in a damp basement or an attic without insulation before it went to the library.

If your book looks like it was water-damaged, or if it has strange spots or stains, it probably has mold growth, experts say. Mold often coexists with mildew. Mildew can appear as a powdery, flaking layer, often white, black or grey, on the surface of the book or pages.

You can’t do much about a moldy library book, other than put it back on the library shelf, but if you’re buying a used book from a library book sale, check out Cornell University Library’s tips for removing mold from books.

Related: The Top Places Mold Hides in Your Home

Bed bugs

Bed bugs and their eggs can hide in the spines of hardcover library books, according to a 2012 report in the New York Times. The report noted some libraries are taking steps to identify the bugs and treat the books that contain them, sometimes with heat.

But keep in mind that you’re much more likely to bring home bed bugs from a hotel or even a movie theater than a library book, according to one 2011 survey of the pest management community.

Viruses and cocaine (yes, cocaine)

Belgian researcher Jan Tytgat of the University of Leuven tested the most-borrowed books from an Antwerp library, looking for bacteria and toxins.

What his team found on the popular erotic trilogy “Fifty Shades of Grey” was a surprise: The books had traces of cocaine and herpes virus, the type that causes cold sores. Fortunately, the levels found were not high enough to be harmful, generally, says Tytgat, a professor of toxicology and pharmacology.

Tytgat puts the finding in perspective, noting that traces of cocaine are more prevalent than you might think. "Whereas it was known since the 50s that each U.S. dollar bill contained traces of cocaine, this phenomenon has now also been seen in the European Union: bills, door handles, covers, books in libraries, clothes, hair — it all shows traces. But this doesn't mean that our society is chronically under the influences of cocaine," he says.

For most of us, no precautions are needed at the library, Tytgat says.

A researcher from a different library book study has changed his ways nevertheless. In that study, researchers from Brigham Young University in Provo, Utah, swabbed books in the library to see whether microbes could survive and spread. The dust collected on the least-circulated books were a magnet for all sources of microscopic life, they found, but more popular books had 25 to 40 percent more bugs than the others. (None of the books had as many microbes as the library doorknobs, which the researchers also tested.)

The study stopped short of figuring out whether the microbes in books might be at a high enough concentration to harm human health. Still, one of the Brigham Young researchers was quoted as saying he has changed his habits. He ''definitely'' washes his hands after handling books, and washes out paper cuts post haste.

Related: Which Has More Germs?

Kathleen Doheny is a Los Angeles journalist specializing in health, behavior and fitness topics.