A treated print is one that's been put through a "rejuvenation" process. This is usually done when a title is reissued. The print is subjected to a chemical bath that cleans the dust & dirt off the surface, and softens the emulsion enough to eliminate scratches and lines. The prints look almost as good as new when projected, and its no doubt cheaper than striking new prints. The only trouble is, 20 years later the acetate base usually breaks down, releasing acetic acid (the dread vinegar smell) and gradually dissolving the film. This is not a problem for the distributors, but since treated prints tend to be titles that are popular and/or classic, it's a real problem for collectors. It's hard to find an IB Tech Disney print that hasn't been treated, since they reissued their animation titles regularly. So of course, these are the prints that are getting smelly and unrunnable. Same problem with those old IB musicals. Some people believe that IB prints are more prone to Vinegar Syndrome than others, but maybe that's because they were treated and reissued more often.
I've heard of all sorts of possible solutions for this problem, from Johnson's Wax to ion absorbtion compounds, but I haven't heard of any positive results. If anyone knows of a treatment that stops or reverses Vinegar Syndrome, please post it!