>One of the hidden costs of bad code is that it is contagious.

That's why the debt analogy fits so well - because technical debt compounds. If your codebase is already heavily in technical debt, when developing new features or fixing bugs you are faced with three equally unpalatable options:

1) Do it properly (takes: 1 month)

2) Don't make it worse; don't make it better (takes: 1 week)

3) Just do a quick hack to work around all of the other hacks (takes: 30 minutes)

As a developer with a pushy boss who doesn't understand technical debt and a tight deadline, the rational choice is pretty obvious: take 3, go home at a reasonable hour to your family, still meet the deadline and drown your professional sorrows in alcohol.