Precautions against malaria are essential, since it remains the big killer in tropical Africa today. Contracting AIDS through unprotected sex remains high as in any other place. Hepatitis A can be avoided by not drinking water liable to be contaminated. Sodas are a good alternative. However, on the whole, Kenya is a safe country to visit, health-wise.
The same cannot be said at all times for safety conditions in Kenya. Nairobi and Mombasa have formidable reputations as being some of the worst cities in Africa for petty crime and armed robbery. Tourists are particularly susceptible to theft and cheating—be sensible and lock your belongings whenever you leave them anywhere. Do not carry expensive articles of electronic equipment/jewellery with you when you walk around the city. It is best to avoid walking around after dusk in the big cities—take a taxi. Keep an ear and eye open for local information about crime. You should be especially careful not to fall for any hard-luck stories told by tricksters. Women travelers would be well advised to dress modestly like the locals and in the event of harassment, avoid reacting in any way to comments/gestures—any reaction will be interpreted as a sign of encouragement. However, the locals are usually ready to help, so do make a noise in case of serious trouble. This holds true for all travelers, so there is no need to get overly concerned or paranoid about your safety in Kenya.