it could spread with that fan, mold/mildew will spread spores, the spores will grow anywhere simulating the right conditions, and you will have a problem, get an organic fungicide, check the back to see if it can be used in flower, and check if it is suitable for use on food crops, if it isnt, dont use it...

if not, spray the bad leaves with water, and rub as much of it as you can off..

good luck...