Sri Lanka was certified malaria-free by the WHO in 2016, which simplifies planning considerably. You do not need malaria prophylaxis.
Recommended vaccinations before visiting include Hepatitis A, Typhoid, and routine immunizations (tetanus, MMR). For longer stays or travel to rural areas, consider Hepatitis B and Rabies. Consult a travel health clinic at least four to six weeks before departure.
Dengue fever is a real concern. It is transmitted by mosquitoes that bite during the day, and cases are reported year-round across the country. Use repellent with DEET, wear long sleeves and trousers in the early morning and late afternoon, and sleep under a mosquito net if your accommodation does not have screened windows. If you develop a high fever within two weeks of arriving, see a doctor.
Water from taps is not safe to drink. Stick to bottled or filtered water. Most guesthouses provide filtered water for free or for a small charge; ask before buying. The same applies to ice — be cautious in local restaurants, fine in tourist-oriented places.
Medical facilities are reasonable in Colombo and major cities. Nawaloka and Asiri hospitals in Colombo treat foreigners and have English-speaking staff. Outside the cities, facilities are limited. Travel insurance with medical evacuation coverage is strongly recommended.
Heat and sun are serious risks in the lowlands and at archaeological sites like Sigiriya, where you will be climbing exposed rock. Start early, carry water, and cover up.
Pharmacies are well-stocked in cities and most towns, and many medications available only on prescription in the West can be bought over the counter at low cost.