I just read about the Xavier malware that has apparently "infected" some apps on the Google Play Store.
As usual when there are news like this, they dumb it down to a level that most people can relate to, using terms like "infected with" etc. Some newspapers make it sound as though there is a virus running amok in the Play Store, "infecting" apps one by one, which I find highly unlikely! The article I linked to does seem to indicate that the malware is actually a third party ad library used by the affected apps.
I'm a developer myself, and I don't really get how an app could be "infected by" a library. A library is something you actively choose to include in your app because it provides functionality that you want. Do they mean that the library maker purposely included malicious code in their ad library, or was the library somehow "infected" by some unknown actor?