I know from balancing the exposures of 9-up and 16-up lensboards for package printers that you can put something opaque, like model airplane dope, on a lens and still get a perfectly acceptable print. The dot of fluid dries and behaves in much the same way as a secondary mirror in a catadioptric system, which is to say you get donut-shaped out-of-focus areas and you lose a little light and contrast. Fungus should do more or less the same thing unless it's really bad.
About ten years ago, I gave a Tamron 70-210/3.5 that I wasn't using to a friend from college. She stopped using it and it got stored in god-knows-what conditions for several years. I got it back last November when she was cleaning out her packing after moving. The lens was covered in fungus, inside and out, with maybe four or five internal surfaces affected; it had very low contrast overall, much lower than when I used it regularly. It was still usable unless you put it in a situation where it would flare, in which case it was horrible. I think it depends on how much fungus there is, and where it is, along with the lighting in the image and how much you stop down (large apertures being better due to DOF, I suppose; I didn't really try any sort of scientific experiment to figure it out, because I have a Nikon Series E 70-210/4.0 that is a better lens IMHO).
