I bought a used Sigma lens on ebay for my back up camera and it was said to be in "mint" condition. I got it today and look it out of the box to look at it and I was horrified to see that the front glass looks like it has frost inside it! Is this fungus? I have never had or seen fungus before. If this is fungus can it contaminate my camera or infect my other lens if I switch them out?
The photograph looks like dust, but if you say it looks like snowflakes then yes, that is a fungus. See http://forum.mflenses.com/fungus-on-lens-t1301,start,105.html for examples and send that lens back/complain!
Your description and pic suggests that your lens has fungus. The snowflake suggests a web like spread, synonymous with Fungus.
In general, fungus does not spread from one lens to another. However, the likelihood of 2 lenses getting fungus if they are both stored together in a dust filled humid environment; is very high. To germinate, these fungus spores inside the lens, which incidentally are everywhere, need the humidity and the dust to feed on.
Without these ideal conditions, the spores will not have anything to feed on and thus not spread and create further spores within the same lens.
You will find that by storing the lens in a dry dust free environment, the fungus will not spread. You may also at this stage find that the lens is giving what you will consider a sharp and satisfactory image, however, if you have paid good money for the lens and were expecting a Mint Product, and also if the thought of having Fungus on your lens is bothering you, then for your peace of mind, a refund is the better solution.