Web Advertising: How one advertisement tarnishes the rest

Have yet to come across a better explanation on how a single advertisement on the web tarnishes the rest.
Web pages are like fast food hamburgers, where a single patty might contain the blended meat of hundreds of cows spanning four countries. In the fast food context, one contaminated carcass is reported to be able to pollute eight tons of ground meat. For the Web, a single advertisement contaminated with bad code can instantly be circulated to those browsing tens of thousands of mainstream Web sites operated entirely in good faith.