I've noticed that researching diseases does mean they're less likely to crop up. But I usually only do the first level and my zoos stay pretty much disease-free so far. Cross-fingers.
Heh-heh, I've always chuckled at the percentages described for the research steps. 250% lower chance of spreading the disease? 100% would mean never spreading it all, so what's with the 250%?
To me it sounds like being 250% pregnant.
(Yes, I realize this is a trivial item in the grand scope of things, it just strikes me as funny)