Meaning: a generic disease, often used to describe flu of various sorts.
Usefulness: 1 (When you have one, as I do. Doctors talk vaguely about viruses, but we all know they mean lurgi.)
Logofascination: 1 (Possibly invented by the Goon Show, this word has been particularly useful to diseased bloggers: the Inky Fool considered its possible origins, and Lynneguist linked to a comprehensive World Wide Words post and a video on cooties. I'm interested in the spelling issue - the Goons spelt it lurgi, as I normally do, but the OED has lurgy. Here's hoping those links will keep you busy, as I may need a night or two off to do battle with it.)
In the wild: Mainly in the UK, and dying out in books, if ngram is to be believed.