well, to be fair, there was a time when i didn't know what fungus was.. but it happened once only.
anyhow, this is what fungus looks like, this is heavy stuff, i think it would be not so common unless the guy has really abused it badly:
the next time you can look up through it at the light, you should be able to see something like this:
http://jameslimphoto.com/blog/wp-content/uploads/2008/06/img_3629a1.jpg
which is a more common degree..
spiderweb patterns, etc. basically something that looks organic.
well, it could be dust as well, i am not sure.. but in any case it is not a good thing to be present on the lens, and perhaps letting people know so that they won't get ripped off is a good thing.