The very publicized case of Kimberly Bergalis, who might have contracted AIDS as a result of her dentist (Florida?).
I seem to recall there was some discussion that maybe it was not carelessness on her dentist’s part, but he might have been deliberately infecting people.
It’s been a while, and I don’t know whether anyone ever came up with “the most plausible” explanation–that is, accidental or deliberate.
I think all the parties involved are dead, now, anyway.