Africa is a big place, so diseases vary widely over the continent. In some places the risk of disease is no worse than in the United States; other places have issues you absolutely need to prepare for. The best course of action is to visit with your doctor or a special travel doctor months before leaving. Inform them of the countries to be visited.
The Center for Disease Control (CDC) keeps an updated medical file for each country and is a good resource, especially for info on travel immunizations and medications for Africa. It is common to require booster shots for existing vaccinations and for certain places malaria medication is required. Be advised that some malaria medications require advanced consumption to be effective, so it is vitally important to visit with your doctor early.
Be sure to take along any prescriptions required. It is not a bad idea to also have your doctor prescribe a medication for traveler's diarrhea. The food and water in Africa is generally fantastic (however, stomach sickness can occur anytime you travel), but having something to deal with it can make your trip much more pleasant.
As a final note: Some people don't take sleeping medication for the flight, but I have found it helpful in preventing, as well as reducing, the effects of jet lag. If you can force yourself to sleep during the 18-hour Atlantic crossing, it not only passes the time, but also puts you in a much better position once you land on the Dark Continent. If you think it will help, see a doctor for consultation.