HEALTH

Minimizing gastrointestinal problems
Since most of Nepal still gets along without modern sanitation, these are endemic. They range from self-limiting attacks of diarrhea where dehydration is the main risk, through intestinal parasites, amoebic dysentery and giardiasis which are chronic without proper medical treatment, to immediately life-threatening infections like cholera and typhoid. Habituation even to common intestinal flora generally takes about a year and many unpleasant bouts of stomach problems, so tourists contemplating shorter stays should take extensive precautions.

Drink only bottled water or juice (check to ensure the seal has not been broken) or beverages made from water that has been thoroughly boiled and filtered. Tea or coffee from cafes catering to tourists are 'generally' safe. When trekking carry iodine or other chemical means of treating water and be sure to follow directions, i.e. don't drink the water before the specified time interval to ensure than resistant cysts are deactivated. In trailside teashops, although glasses may be washed in questionable water, tea is made by pouring boiling water through tea dust into your glass. The chances of disease-causing organisms surviving that are small but not zero.
Brush teeth with prepared drinking water and avoid water entering the mouth when showering.
Salads, especially in the wet season, should be treated as a suspect.
Wash hands regularly and especially before eating.
Thoroughly wash fruit and vegetables for raw consumption using boiled and filtered water. Also consider peeling them.
Look for freshly-cooked food and avoid anything that has been cooked and then left sitting around without refridgeration (which can expose you to a buildup of bacterial toxins), or without protection from flies (which can transfer disease organisms and parasite eggs to the food).

Get vaccinated and consider prophylactic treatment.
Malaria appears only in rural areas at altitudes below 1200 m (3937 ft) in the Tarai and Hill districts bordering India and for the areas of the inner Tarai valley areas of Udaypur Sindhupalchowk, Makwanpur, Chitwan, and Dang. It's recommendet to carry Lariam, Malarone, or doxycycline with you to treat this disease when you are infected.

Hepatitis A vaccine is recommended for all travelers over one year of age. It should be given at least two weeks (preferably four weeks or more) before departure. A booster should be given 6-12 months later to confer long-term immunity. Travelers who allergic to a vaccine component should receive a single intramuscular dose of immune globulin in the place of vaccine.
Typhoid vaccine is recommended for all travelers.
Polio vaccine is recommended. Any adult who received the recommended childhood immunizations but never had a booster as an adult should be given a single dose of inactivated polio vaccine. All children should be up-to-date in their polio immunizations and any adult who never completed the initial series of immunizations should do so before departure.
Hepatitis B vaccine is recommended for travelers who will have intimate contact with local residents or potentially need blood transfusions or injections while abroad, especially if visiting for more than six months.
Tetanus-diphtheria vaccine is recommended for all travelers who have not received a tetanus-diphtheria immunization within the last 10 years.
Cholera vaccine is not generally recommended, even though cholera occurs in Nepal, because most travelers are at low risk for infection. This vaccine is recommended only for high-risk individuals, such as relief workers, health professionals, and those traveling to remote areas where cholera epidemics are occurring and there is limited access to medical care.
Yellow fever vaccine is required for all travelers arriving from a yellow-fever-infected country in Africa or the Americas, but is not recommended or required otherwise.

Altitude sickness
Permanent snow lines are around 18 or 19,000' (5,500m), so base camps and passes in the Himalaya are usually higher than Mount Blanc or Mount Whitney. This puts even experienced mountain climbers at risk of altitude-related medical conditions that can be life-threatening. Risks can be minimized by choosing routes that don't go high, such as Pokhara-Jomosom or by sleeping not more than 1,000'/300m higher per day and to take an experienced guide like me if you plan to go in higher areas. According to the "climb high, sleep low" mantra, it is good to take daytime conditioning hikes that push acclimation, then to return to a more reasonable elevation at night.

Read more about this topic here

Hypothermia
is a risk, especially if you are trekking in spring, autumn or winter to avoid heat at low elevations. When it is a comfortable 85F/30C in the Terai, it is likely to be in the teens Fahrenheit or -10C at that base camp or high pass. Either be prepared to hike and sleep in these temperatures, or choose a trek that doesn't go high. For example at 10,000'/3,000m expect daytime temperatures in the 40s Fahrenheit or 5 to 10 degrees Celsius.

Rabies
Dogs are not vaccinated and catch this fatal disease from other dogs or wild animals with some regularity. All mammals are potentially vulnerable. Dogs are considered ritually polluting and are widely abused, so it can be impossible to know whether a dog bit you because it is paranoid about people or because it is rabid. You should be vaccinated against rabies before going to Nepal, but this is not absolute protection. Be on the lookout for mammals acting disoriented or hostile and stay as far away as possible. Do not pet dogs, cats or pigs no matter how cute. If bitten or exposed to saliva, seek medical attention. You may need an extended series of injections that provides a higher level of protection than routine vaccination.

Snakebite
The risk is greatest in warm weather and at elevations below 5,000' (1,500m). Poisonous snakes are fairly common and cause many deaths annually. Local people may be able to differentiate poisonous and non-poisonous species. Cobras raise their bodies in the air and spread their hoods when annoyed; itinerant snake charmers are likely to have specimens for your edification. Vipers have triangular heads and may have thick bodies like venomous snakes in North America. Kraits are strangely passive in daylight but become active at night, especially around dwellings where they hunt mice. Krait bites may be initially painless, causing only numbness. However without proper antivenin numbness can progress to deadly paralysis, even with bites from small, seemingly innocuous specimens. Wearing proper shoes and pants rather than sandals and shorts provides some protection. Watch where you put your feet and hands, and use a flashlight when walking outside at night. Sleeping on elevated beds and on second stories helps protect against nocturnal kraits.