I found an answer on the title.

https://www.reddit.com/r/AskHistorians/comments/1o3uek/the_oatmeal_just_released_this_post_about/ccowutk/

I'd still like an answer for this

If the answer to the first question is yes, did the slave traders consciously know about it? Was there scientific research about immunity?

edit: The bot told me to tag u/khosikulu and u/Killfile .