In the 2008 Report on the Global AIDS Epidemic the UNAIDS/WHO Working Group estimated that around 5,400,000 adults aged 15 or over in South Africa were living with HIV; the prevalence rate was estimated at around 18.1% of the adult population. This compares to the prevalence rate in adults in the UK of around 0.2%.  You should exercise normal precautions to avoid exposure to HIV/AIDS. For more general information on how to do this see HIV and AIDS.

Though effective, antibiotics are not recommended prophylactically (i.e. to prevent diarrhea before it occurs) because of the risk of adverse effects, though this approach may be warranted in special situations, such as immunocompromised travelers.

Malaria in South Africa
Prophylaxis is recommended for travel to low-altitude areas of the Mpumalanga Province (including Kruger Natonal Park), Limpopo Province, Northern Province, and northeastern KwaZulu-Natal as far south as the Tugela River. Risk is highest from October to May. Either mefloquine (Lariam), atovaquone/proguanil (Malarone)(PDF), or doxycycline may be given. Mefloquine is taken once weekly in a dosage of 250 mg, starting one-to-two weeks before arrival and continuing through the trip and for four weeks after departure. Mefloquine may cause mild neuropsychiatric symptoms, including nausea, vomiting, dizziness, insomnia, and nightmares. Rarely, severe reactions occur, including depression, anxiety, psychosis, hallucinations, and seizures. Mefloquine should not be given to anyone with a history of seizures, psychiatric illness, cardiac conduction disorders, or allergy to quinine or quinidine. Those taking mefloquine (Lariam) should read the Lariam Medication Guide (PDF). Atovaquone/proguanil (Malarone) is a recently approved combination pill taken once daily with food starting two days before arrival and continuing through the trip and for seven days after departure. Side-effects, which are typically mild, may include abdominal pain, nausea, vomiting, headache, diarrhea, or dizziness. Serious adverse reactions are rare. Doxycycline is effective, but may cause an exaggerated sunburn reaction, which limits its usefulness in the tropics.

In any case, you should seek medical advice before travelling to South Africa and ensure that all appropriate vaccinations are up-to-date.  For further information on vaccination requirements, health outbreaks and general disease protection and prevention you should consult your personal physician or a travel health clinic 4-8 weeks before departure.