The biggest medical threat to visitors to Rwanda is malaria, with the risk of transmission being greatest at lower altitudes and during the rainy season. A variety of oral prophylactics against malaria is available, so before you travel, you should seek up-to-date advice about the most suitable option from a travel clinic or doctor.
No prophylactic is completely effective, so try to avoid being bitten by the nocturnal Anopheles mosquitoes that transmits the disease. Cover up in the evening, by wearing a long-sleeved shirt, trousers and socks, and apply a good insect repellent clothes to any exposed flesh. When you retire, sleep under a net, or failing that in an air-conditioned room, under a fan, or with a mosquito coil burning.
Malaria usually manifests within two weeks of being bitten, but can take months, so if you display flu-like symptoms after you get home, get to a doctor and ask to be tested immediately. Travellers with young children or who prefer not to take medication might consider visiting a malaria-free safari destinations elsewhere in Africa.