Your medical adviser can tell you how to prevent mosquito-borne illnesses like malaria. Take the necessary things with you. Hepatitis C is a blood-borne virus that can spread through skin puncture, a toothbrush, razor or even tweezers. Practise safe sex. Pack a supply of reliable condoms and water-based lubricant.
Some overseas condoms are sub-standard.
Find out before you go. Some countries bar entry of HIV-positive travellers. Ask the embassy of the country or your State AIDS organisation. Take with you a letter from your doctor or dentist describing your condition and your medication and make sure you have sufficient supplies. If your medication requires injection, purchase only unbroken sterile packs of needles and syringes, or carry your own supply if acceptable to the country you are visiting.