Read up on the various medicines....both preventive and therapeutic....and you will find there is no guarantee you will not contract Malaria. The Mozzies carry Malaria, expose yourself to them and you are exposed to Malaria. It seems about like the Lottery....some guys contracted Malaria and others did not. Some used Pills...most did not except as a Theraputic cure rather than as preventive measures.
There have been a couple of fatalities from Cerebral Malaria....so it is an important issue to consider but not one that should prevent you from working in Nigeria. Gulder probably presents a much worse danger of bodily harm.