According to Neville why are bullets ineffective in killing vampires? I Am Legend by Richard Matheson

From the text:

Through experiments on the dead vampires he had discovered that the bacilli effected the creation of a powerful body glue that sealed bullet openings as soon as they were made. Bullets were enclosed almost immediately, and since the system was activated by germs, the bullet couldn’t hurt it. The system could, in fact, contain almost an indefinite amount of bullets, since the body glue prevented a penetration of more than a few fractiosn of an inch. Shooting vampires was like throwing pebbles into tar.