Err... You mean the malaria that they brought from Africa? And this is more of a problem in the Americas than in Africa, why?Lastly, West Africans were still susceptible to diseases in the Americas. They were just a little bit less likely to die than Europeans. Malaria should still be an issue.
The only disease from the Americas that became epidemic in the Old World is probably syphilis.
Edit: Mali creating a colony (using the definition of: a Malian based settlement 1 or 2 boat loads of people who fetch up in the Americas) is easy. Creating a colony (using the definition of : a dependent settlement politically under the control of the motherland and with significant trade) is very, very hard as Blackfox5 pointed out, unless their ship building tech is a whole lot better. no?