I found a recently published article (September 2010), which presents information on research regarding the evolution of malaria. Scientists have concluded that the parasite most likely originated in the lowland gorillas, who may be infected with a "genetically identical microbe." Amazingly, researchers also believe that the parasite crossed the species border with just one mosquito bite--one bite is responsible for the outbreaks and epidemics of the disease!
Furthermore, scientists are hopeful as to the benefits of knowing the source of the parasite. They believe that with this new information they may be better able to understand and treat the disease. The article uses HIV as a parallel study, because knowing that HIV originated in primates has allowed for further research into the nature of the disease.
Finally, the researchers believe that the parasite does not have as negative an effect on the gorillas as it does on humans.