The problem with the 'infection' metaphor is that infections are things that spread on their own, against the person being infected's will.
When a library is under the GPL, that's not an infection, it's a price -- you can use the library if you pay it back by freeing your own software in return, or you can not use the library and not pay that price. Totally up to you.
Ironically, some of the people who hate the idea of this kind of quid-pro-quo rant about how it's 'communist'. (And, to drive the point home, also hate the first-sale doctrine, traditional contract law as applied to EULAs, etc.) Really it's 'capitalism' they seem to object to.