I've had some experience with fungus on old glass before, here's my experience:
Bought a set of 3x Zeiss lenses for Pentacon Six off ebay, I knew it was a gamble but the price was right, all 3 had fungus in some way.
The 180/2.8 Sonnar had fungus internally, around the inside of the front element.
The 50/4 Flektogon had it about in the centre of a middle element.
The 80/2.8 Biometar had both dust and fungus in the centre of the element just in front of the aperture.
Firstly, they all went in the sun for a few days to kill all the spores (Be careful, I nearly set fire to my lawn), aperture as open as possible.
So, the Sonnar 180 only had fungus around the rim. It's got a 6*6cm medium format image circle, and an 82mm filter ring. I can attach step-down rings to 58mm without vignetting on my 7D. The fungus was nowhere near the image circle on APS-C. What camera do you have? If you're on APS-C or even APS-H and the fungus is right near the rim only, you could be lucky if you kill it properly and it doesn't spread.
The Flektogon 50 I took the rear few elements off, with a blower and microfibre cloth I managed to clean all the dust and spores out, this lens works fine now.
The Biometar 80 I also took the rear elements off, but the screw threads were very sticky and it took a lot of straining. Cleaned it off alright, the fungus had etched into the glass and multicoating around the edge but nowhere near the APS-C image-circle. But then I put it back together, and couldn't line up the elements exactly. So now it's got a bit of a "soft-focus" effect wide-open, still sharp as nails at f/5.6 or so. I don't use this very much anymore. I could probably try playing around with the element spacing to get it back to sharp at f/2.8, but I could just as easily buy another off ebay for $50-100, and i'm too lazy to even do that now that i've got an EF 85/1.8.
Otherwise, you'll have to inspect it closely, if it's just fungus it's fine, if it's etched into the coatings you just get a bit more flare, but if it's etched into the glass then you 'could' start to see it affect IQ. You can also take a gamble, if it's only one lens element affected, then maybe you can strip it for parts and make a bit of cash, but not if you want to actually use it.