"He has a virus in the chest."
Translation:Han har en virus i brystkassen.
I think it relates to the fact that you can only have one temperature or fever. There are many vira to choose from. It is one of those vira that is in question (although it may not have been revealed which). That's my best explanation, I hope it makes sense? If not, the answer will be: Because Danish! ;-)
I know. But it is still poor. He should've said "in" as you suggest. You also sometimes hear people in clothing stores talk about "en buks" ("a pant"). The fact that it is said does not make it proper language but it will be understood by all. Most often people saying it don't know neither care if it's right or wrong.