Found this site on a google search. I do know off hand there area a couple diseases rats can give to humans (though not all rat to human diseases are carried by domesticated rats. Healthy vet inspected rats are unlikely to have these diseases to start off, so its always good to buy healthy stock (even if they tend to cost more).
www.aaanimalcontrol.com/ratdiseases.htm
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PHLdyPayne

