Malaria is the biggest single medical threat to visitors to Kenya. It is present in most parts of the country throughout the year, though the risk of transmission is generally far higher at low altitudes and during the rainy season. There is no vaccine, but several different oral prophylactics are available, and it is advisable to visit a travel clinic or other suitably qualified medical professional for up-to-date advice about the option best suited to your requirements. No prophylactic is 100% effective, so take all reasonable precautions against being bitten by the nocturnal Anopheles mosquitoes that transmits the disease.
These include donning a long-sleeved shirt, trousers and socks in the evening, and applying a DEET-based insect repellent clothes to any exposed flesh. Always sleep under a net, or failing that in and air-conditioned room, under a fan, or with a mosquito coil burning. Malaria normally manifests within two weeks of being bitten, but it can take months, so if you display possible symptoms after you get home, get to a doctor immediately, and ask to be tested. Travellers with young children or who prefer not to take medication might consider visiting a malaria-free safari destinations elsewhere in Africa in preference to Kenya.