Humans apparently first got malaria parasites by being bitten by mosquitos that had also bitten western gorillas.
Loy et al. 2017.
Out of Africa: origins and evolution of the human malaria parasites Plasmodium falciparum and Plasmodium vivax

That species of gorilla is nearest to the island of Bioko. Using DNA of parasites, viruses, etc., the origin of humans on Bioko is a testable hypothesis, not an unprovable model. When geneticists become aware of this hypothesis, they will begin actively testing it. 

Here is an interesting paragraph from that paper: 

Whilst it could be argued that the ape P. vivax was brought to Africa by humans who migrated from Asia (Prugnolle et al., 2013), this hypothesis has been refuted by sequences indicating the existence of a related, but distinct, Plasmodium sp. that also infects African apes. This Plasmodium sp. which is apparent in trees of mt, nuclear and apicoplast sequences (Fig. 4), has been found in chimpanzees from two different locations in Cameroon (the BQ and DG field sites in Fig. 2) and represents the closest known relative of P. vivax. The most parsimonious interpretation of this finding is that the common ancestor of these two species was in Africa, indicating that the lineage existed there for a long time before P. vivax arose as a distinct species (Fig. 4). We propose to designate this newly described species Plasmodium carteri, in honour of Richard Carter, who has long championed the hypothesis that P. vivax originated in Africa (Carter, 2003; Culleton and Carter, 2012).