Is fever a disease or not?
I know pretty much nothing in medicine!

Fever is an abnormally high body temperature. Fever is a symptom and not a disease per se. Different medical conditions (incl. various infectious diseases) causes a patient to have fever.

Note that fever might be in some/most situations an adaptive response to a disease. In other words, for infectious disease causing fever, fever might be good for the host and not for the parasite. However this issue remains controversial (see the post linked in the OP's question for more info).

As GoodGravy said in the comments, many people get confused and consider fever as a disease instead of what it really is; a symptom. One likely reason for this confusion is that when one has fever, (s)he very often treat fever directly (taking drugs like Aspirin for example) rather than treating the underlying cause (the virus for example) of the fever.