It's been noted that African - and Italian - populations have genetic defences against the Malaria parasite. Plural: the famous one is sickle-cell, but older mutations exist. These do not lead to the anaemia. Why haven't they led to the anaemia?
Article here has a suggestion: some genetic tweaks are not allergenic, but antiallergenic.
To me this suggests a high-Eurasian pathway similar to the Saharan pathway. People move to new environments. Those environments have not mutually adapted with the newcomers. The environment resists. The newcomers mostly die - but not all. If going home is more hazardous than toughing it out, the survivors will tough it out. Those will stay - but weaker. Weaker relative to their relatives back home; but it's not like those relatives are going to follow them into the swamp, they're not idiots.
- until the body gets countermeasures. Fast-forward thousands of years. Some in the next generation innaswamp are no longer so weak. They got the original antimalarial genes, and new genes to work around all the problems the original genes had raised.