Brooks uses the analogy of a silver bullet that kills the werewolf to describe something in software th

Brooks uses the analogy of a silver bullet that kills the werewolf to describe something in software that makes software costs drop as rapidly as computer hardware costs do.

So, a silver bullet is the enemy of a werewolf, because it can easily kill it. Similarly, there can be something, like a virus, in a software, that will bring about issues in a software, and thus its price is going to drop given that it is a faulty product.