I've found an old 80mm Macro lens for my Mamiya 645. The lens however has a small amount of fungus along the edge.

Can someone help me understand lens fungus. Will it affect image quality? will it spread?

Check out this link (Google is a wonderful thing):

http://www.chem.helsinki.fi/~toomas/photo/fungus/

We have an especially vicious kind of fungus here in FL, from what I understand. So far, I have not had to experience it (knock on wood-like substance nearby).

I live in South Florida and i've lost thousands of dollars worth of lenses over a period of 25 years or so. I've sent lenses to the makers, canon, tamron and sigma and they basically say toss them out. Once the spores start growing in a lens, no matter what they do, they can't kill them all and they just grow back. I try to be carefull how i store them, in a/c. Good luck and take care with your lenses in a tropical climate.

I'm in FL, too, and so far none of my EOS lenses have been infected. But, I do have a 30 year old Minolta 50/1.7 and it did have quite a bot of fungus after sitting unused for ~15 years in a dresser drawer. So, 4 or 5 years ago I disassembled it, cleaned all the elements and barrels with alcohol, reassembled it, and it's been clean ever since. I've heard about fungus actually etching glass, or maybe it's the coatings, but looking at the elements under magnification, I couldn't find any damage.