Diseases don't just randomly spawn because you have urban areas. Cities are rife with disease because of things like crowding, poor sanitation and other factors making it easier for what's there to spread.
And I'm missing why they'd have better immunity to diseases they have no exposure to.
So we're going from "Mali makes contact" to "In the year X+2, the Mali sailed across the ocean blue" - bringing Old World diseases to do their deadly work even earlier than OTL.
So the fact Eurasians have the worlds worst diseases, and no one else has anything even comparable is all luck, thats what your saying?
Yeah, but trade with Mali would reap nowhere near the destruction colonization left.
Okay, lets get off the fact of colonization for a minute. When do you think the most likely time for fish farming to start would be? BC or AD? BC would probably be preferable for technological growth, But they could catch up if it was AD considering the period of next to no growth that Europe went through, -cough - Dark Ages -cough-