Given the analogy you are using, do I understand it correctly that software can "catch" GPL (as in being required to be released under the GPL, I suppose?) with

Given the analogy you are using, do I understand it correctly that software can "catch" GPL (as in being required to be released under the GPL, I suppose?) without a deliberate decision of its developers to include GPL licensed code into their codebase? Could you explain how that works? Also, would you use the same language to describe typical proprietary licenses? After all, most of those would also prevent me from publishing any codebase incorporating any code I obtained under such a license as free software--or in your terminology, the codebase would be "infected" by the proprietary license which forces my software to also "catch a proprietary license"? I'm not quite sure I understand your analogy, though ...