Staying healthy requires proper preparation before departure and self-discipline in-country.
You should also bring a small medical kit suitable for West Africa. Be sure to include an antidiarrheal, like loperamide, as traveller's diarrhea is by far the most common complaint.
In addition to routine childhood and adult vaccines (see wwwnc.cdc.gov/travel/diseases/routine) these vaccines are specifically recommended for travel to STP:
After years of eradication efforts, the risk of malaria in São Tomė is low, and extremely low in Príncipe, which at time of research had entered the pre-eradication stage. Antimalarials are still recommended for São Tomé; travellers to Príncipe only should check with their travel medicine provider to see if they are necessary. Risk is at its lowest everywhere during the dry season.
Your doctor will prescribe the right antimalarial drug for your situation. In general, doxycycline and Malarone are relatively problem free, whereas mefloquine has some unpleasant psychological side effects while being no more effective than the alternatives.
Healthcare in STP relies on foreign aid, including a large medical mission from Taiwan. Expats turn to the Gnostica Medical Center, where a normal appointment runs €10. The São Tomė city hospital should be avoided.
Only drink bottled water in STP.