The term “bookworm” is used in two senses. The first refers to any type of insect which infests books, while the other refers to a person who enjoys books. The second may be used pejoratively, suggesting that the person has become obsessed with books. In the second sense, avid reading can certainly be hard on books, especially cheap paperbacks, but it is generally not as potentially damaging as insect infestation. The intended meaning is usually made clear by context.
The first usage of bookworm can get rather vague. No species of insect is specifically known as a bookworm, although a wide range of insects from silverfish to termites will eat books and paper, if the material is available. The larvae of several insects are also rather fond of books, especially the glue used in older books, and some of these larvae will tunnel into books. A true book borer, however, is rather rare.
Controlling insects in a library can be a problem, especially in an older library, which may be congenial to insects and damp as well, posing a serious significant risk to the contents. Most bibliophiles try to keep valuable books in controlled environments, where they are less subject to infestation. Keeping a library clean and dry can also help. An abandoned library, however, can be subject to large amounts of insect damage, especially in the tropics.
The second usage of bookworm actually predates the first; as early as 1599, people were referring to book lovers as bookworms. The term was only applied to insects in the 1800s. Some people prefer to distinguish bookworms from bibliophiles, arguing that bookworms love books for their contents, while bibliophiles love books as objects. Clearly, some overlap probably exists between the two, as plenty of bookworms collect old or beautiful books, and many bibliophiles greatly enjoy reading.
Different people have different standards about bookworms, often determined by their own reading habits. The term is often applied to children, especially shy children who spend much of their spare time reading. Adults, however, can certainly be bookworms as well, especially when they have a great deal of spare time on their hands. If you cannot leave the house without a book, you might be a bookworm. This is especially true if you pop the book open at every opportunity, or if you have been known to read while walking down the street, cooking, or performing similar tasks.
