Kenya has nasty malaria strains. Seasons matter, but I always take prophylaxsis when I make short trips.

Hi - locals will not bother taking malaria tablets for Kenya, particularly in Nairobi. The altitude in Nairobi and the central plateau is high - something like 8000ft, and mosquitos are not in abudance, and malaria is rare. The coast is different, there are mozzies everywhere in certain seasons, and malaria is more likely. Your doctor will certainly advise you take malaria tablets, but they have to, my advice is dont bother if you are just going to Nairobi (or surrounding area) but do take malaria tablets for Mombassa and the coast. Whatever, use mosquito repelant and mozzie nets at the coast, and see a doctor immediatly if you get flu-like (malaria) symptoms. Hospitals in Kenya are probably better at dealing with Malaria than those in London, suprisingly.