If you are only going along the main mountain trekking routes and not hanging out in the Terai,staying at farms with animals, living and eating out of poorly sanitated rural homes, I would not be concerned about most exotic diseases and parasites connceted with Nepal.
There are probably more MDs along the national park trails than anywhere else in Nepal. If something really serious happens there are rescue services and what not available in these areas. However, all rescue, evacuation and medical services are built on the premise that you eventually cough up the money it costs. Thus, what you really need is a comprehensive travel insurance that is valid for expedition style activities, and includes helicopter evacuation, not solely a medical insurance. Most common travel insurances are invalid above 4000 meters altitude, and you need to add an expedition clause + cost to it.
Prevention is much, much better than cure. Eat local food. Do get all vaccines needed for the particular trek (again, you can skip some of the farm/animals/lowland related stuff for mountain treks). I found that health personnell tend to consult the WHO handbook prior to setting your vaccine program, and the private clinics earn money for each vaccine, so even qualified medical personnell may not be completely up to it on the local conditions.
Must-have vaccines are: typhoid, Hep-A, diptheria, polio, tetanus, meningitis, BCG (tub), and the cholera drinks (Dukoral etc). The cholera vaccines are not very effective against cholera, but very effective against some other common diarrheas. No malaria or kala azaar or other nasty stuff on the mountain routes, unless you already are a carrier, or dipping toward 6-700 meters.
Regarding parasites, ghiardia is a real nuisance that you can bring antibiotics for, several brand names, most of them contain metronidazole. This is a prescription drug, so take it with great care/correct dozage and only if you have diagnozed ghiradia. Of all stomach upsets, I guess only 10-15% are caused by ghiardia. I can't see the high mountain routes are a particular bad territory for catching intestinal worms, but if in doubt and you've eaten roughly around animals and badly washed veggies and uncooked foods, take a series of mebendazole when back home again. I do that upon completion of high-risk trips, having stayed "local".
What will happen to you on the mountain treks? Possible food poisoning, not always preventable. You can even get it from a fresh hard-boiled egg. The best is to stay put and get it over with, hydrate with tea, noodle soup, hot water, and do not push on higher and harder. Immodium I would only use if I really have no outlet - e.g. travelling by plane/bus. Influenza - a real chance among so many people at lodges and travelling in a group. Very debilitating, and it can escalate to lung diseases if you do not take care. Note that typhoid also has influenza-like symptoms. Dry air, dust and cold-related sore eyes, skin, lips, the Himalayan cough (not the bird!), a constant cough that hits people due to climate, dust and altitude. Soar throat comes from the same conditions, I use paracetamol and lozenges to keep it down, but if it is sign of common cold arriving, it's nothing you can do apart from letting it pass as smootly as possible. Sun: sunglasses, sun protection hats, gloves, neck cover, sunblock. Falls; you may stumble and fall, something to take care of bruises and small cuts. Antiseptic cream to put under bandages. Blisters, chafes, sprains, pains in the feet; just do not let it go too far. Keep clean. Unclean water: boil the water, even at altitude, it is suffcient to kill most critters. Use boiled water to fill water bottles at lodges, add a disinfectant pill like micropur silveriodine. Liquid or chrystal iodine is a mess, but super effective. Unless you are much off the trail, I would leave the water filter at home: heavy stuff, slow at work, and out-competed by availability of boiled water. Hand sanitation is probably the single most important thing: wash often, use soap if possible, and use the odd dose of alcohol based antibacterial gel in connection with food intake.
AMS/HAPE - the real killer: go slow, slow, slow. If in a group and the group is uneven at acclimatizing, do extra days at the same spot and the well acclimatized ones can do day trips out to explore while the others catch up. Read up on AMS and HAPE and team up two and two at the beginning of the trip, so you have a mate to keep an active check on you. Diamox kills only the symptoms, and does not prevent AMS. You cannot tell beforehand who is likely to come down with AMS symptoms. At any sign of it, stop and go down again.
The most dangerous thing for visitors to Nepal is car and bus accidents. Once you are out of the highways and cities and are walking on the rural trails you are a lot safer. By the way, once in Kathmandu, check in with the Himalayan Rescue Association for info. They will give you much better info that I can.
You can read dozages and regimens of medicines on labels and from travel medicine books, one of them quoted somewhere on my VT pages. One book cited also gives a diagnostic info sheet on diarrheas. If you really want it I have a write-up on travel medicine, but this is personal advice to those I travel with and are responsible for in my work related trips in Nepal, and nothing really medically professional, even if I have checked it with qualified medicine men, so I will not publish anything like that on VT. But I can send it to you upon request.
Happy, healthy and safe trek!
I wasn't entirely sure which category I should keep this one in: "Must see", "Restaurant Tip" or what.
I decided to post it as a danger-thing, even if it has a certain entertainment value...
The treatment of meat in Nepal is abysmal, and as people have very little concept of why food decays and the infection routes to humans, bacterias and all that, ignorance is meeted out in ample portions when it comes to treatment of meat before it arrives in the pot.
An animal is butchered right on the sidewalk or in the back yard with no hesitation and the meat is often cut in direct contact with soil and other possible contamination. Parasites are very common where people and animals live so closely together. Storage, likewise is a sad chapter.
The good thing is that by cooking the meat very well, most bacteria, viruses and parasites of small and big kinds will succumb to the heat. Certain survivors may persist, though, especially in areas around the bone, where temperatures may be less than required.
Another chapter of the book is called the hygiencic condition of the kitchen and food preparations. I save this for later....
In the countryside it is my impression that the food is generally better handled.
So, what dies this amount to? Certainly, ensure that your meat is very well cooked, so that persistent salmonella will be eliminated, too.... No raw, pink-red, dried or badly cured meat (smelly) of the sukuthi kind unless fried or cooked. And, perhaps, before a long bus journey - don't eat a high-risk meal.
But I would with these precautions in mind splash out on a buffalo sizzler, goat daal bhaat, chicken chilly or grilled fish without hesitation. It's good! And much less fatty than the burger at home!
Since healthcare in Nepal is hard to come across good medical facilities, especially outside of Kathmandu. In the far south, in the Terai, malaria is at a small risk. For the rest of the country, you should be vaccinated for Hepatitis A at least. Make sure to bring iodine tablets for water purification. Aspirin can be very useful to have while in Nepal. Another lifesaver was travelers' diarrhea medicine; make sure to have it if you are spending time in rural Nepal. Try to eat a balanced diet, not so much fried food (even though those momos are so good). In rural Nepal there is only one main food eaten, Daal Baat, which is rice with a lentil curry.
Also try to avoid walking barefoot as much as possible (although almost everyone in rural Nepal walks barefooted) to avoid getting worms.
To stay healthy make sure to drink at least 3-4 water bottles full of water each day.