Laos is a tropical country and health deserves attention, though sensible precautions go a long way.
Water. Never drink tap water. Bottled water is cheap and available everywhere. Ice in tourist restaurants is generally fine; in rural areas, be more cautious. Wash your hands before eating -- the old advice still holds.
Mosquitoes. Dengue fever is present throughout Laos, particularly during the rainy season. Malaria risk exists in rural and forested areas, though it is low in the main tourist towns. Use DEET-based repellent, wear long sleeves in the evening, and sleep under a mosquito net where possible. Consult your doctor about antimalarials if heading to remote areas.
Food safety. Street food is generally safe and delicious if it is freshly cooked and hot. Use common sense -- eat where the locals eat, and avoid anything that has been sitting out. Stomach upsets are common in the first few days; bring rehydration salts and basic medication.
Medical facilities. Hospitals in Vientiane can handle minor issues, but for anything serious most travelers cross the Mekong to Thailand, where hospitals in Nong Khai and Udon Thani are far better equipped. Luang Prabang and Pakse have basic clinics. Travel insurance with medical evacuation cover is essential.
UXO. Unexploded ordnance from the Vietnam War era remains a serious hazard in rural Laos, particularly in Xieng Khouang, Savannakhet, and Salavan provinces. Never stray from well-used paths in the countryside, and never touch unfamiliar objects on the ground.
Vaccinations. Recommended vaccinations include hepatitis A and B, typhoid, and tetanus. Japanese encephalitis vaccination is worth considering if you plan to spend time in rural areas during the rainy season. Rabies pre-exposure vaccination is advisable if you will be far from medical facilities.
Altitude. The northern highlands reach up to 2,800 metres, but altitude sickness is not typically a concern in Laos.