Nepal presents two quite different health environments: the subtropical Terai and the high Himalaya. Both require preparation, but for different reasons.
Altitude and AMS
Acute Mountain Sickness (AMS) is the main risk for trekkers and is taken seriously by anyone who has seen it go wrong. It is caused by ascending too quickly to altitude where the air holds less oxygen. Symptoms include headache, nausea, dizziness, fatigue, and difficulty sleeping. Most people experience mild symptoms above 3,000 metres; severe AMS can develop into life-threatening High Altitude Pulmonary Oedema (HAPE) or High Altitude Cerebral Oedema (HACE).
The only reliable prevention is acclimatisation — ascending slowly and including rest days. Above 3,000 metres, the standard rule is to gain no more than 500 metres of sleeping altitude per day, with a rest day every two to three days. Drink three to four litres of water daily. Avoid alcohol and sedatives at altitude. If symptoms don't improve with rest, descend immediately — altitude sickness does not improve by "waiting it out" at the same elevation.
Diamox (acetazolamide) is used by many trekkers as a prophylactic. Discuss it with a doctor before your trip as it requires a prescription and has side effects. It masks some symptoms and should not be used as an excuse to ascend faster than your body is ready for.
All serious trekking routes have evacuation points, and helicopter rescue is available in emergencies — but it is expensive. Make sure your travel insurance explicitly covers high-altitude trekking and helicopter evacuation to the height you plan to reach.
Terai and Kathmandu Valley
In the lowlands and the capital, the main concerns are food- and water-borne illness. Drink only filtered, boiled, or bottled water. Be cautious with street food, particularly salads, raw vegetables, and anything that might have been washed in tap water. Traveller's diarrhoea is common. Bring rehydration salts.
Malaria is present in the Terai, particularly in Chitwan and Bardia districts, though risk is considered low for short visits in the dry season. Check current recommendations with your doctor or a travel health clinic before visiting. Dengue fever is present in the Kathmandu Valley and the Terai and is spread by daytime mosquitoes — insect repellent is worth using.
Vaccinations
Standard recommendations include hepatitis A, typhoid, tetanus, and rabies (dogs are common and occasionally aggressive). The 2015 earthquake damaged water infrastructure in many areas; hepatitis A immunity is important. Check with a travel health clinic for current guidance as recommendations change.
Medical Facilities
Kathmandu has reasonable private hospitals — CIWEC Clinic and Patan Hospital are well-regarded by the expat community. Outside the capital, medical care is basic. Carry a well-stocked first-aid kit on any trek, and know how to use it.