proper usage of fan:
Fresh air is different from the air allocated in your room. Fan also moves all the dust and fungus gathered in your room onto you/ into your mouth.
Given how the majority of common fungal infections are skinborne and come from contact with infected people, I don't think that this is a very good point. If anything, blowing all the crap from your room towards you would probably build your immune system.
Unless of course, you have aids and/or undergoing treatment that suppresses your immune system/ you live in a humid bog.
The risk of catching anything contagious from your own room is slim if you have even the most basic level of hygiene, however if you're bringing home tramps or stray animals then i guess this risk would increase somewhat