"Transmittable" is the commonly used adjective. " I'd say that if you "transferred" a disease to someone else, you would no longer have it.
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AvangiAs an aside, I've been unable to find a dictionary which doesn't accept "transmittable." Should I continue my search? - A.A Onelook search for "http://www.onelook.com/?w=transmittable&ls=a" hits on 12 general dictionaries, and "