Lonely Planet's guide on Tibet warns of all kinds of horrors: travellers' diarrhea, cholera, giardiasis, amoebic dysentry - and then maladies associated with altitude: acute mountain sickness, different types of respiratory issues - and then the 'normal' warnings like rabies and hepatitis. I didn't quite understand how bleak the picture is that it's painting, until i started googling some of the words.
cat-i: define: Typhoid
google: Typhoid fever is a potentially life-threatening disease caused by Salmonella typhi bacteria. It is usually carried via water or by food contaminated with sewage.
cat-i: define: Giardiasis
google: infection of the intestines with protozoa found in contaminated food and water; characterized by diarrhea and nausea and flatulence and abdominal ...
This went on until I got really scared. So I made an appointment at a travel clinic.
Tetanus: sorted till 2020
Typhoid: fine till 2013
Hepatitis A: have to go get a booster before the end of the year, and I'm sorted for life.
For all the different types of diarrhea, the easiest (and probably most difficult to implement) precaution is to always wash my hands before i touch any food. Bottled water are not recommended (the plastic bottles causes a waste problem in rural Tibet) - but boiling water (even at lower temperatures due to altitude) will kill anything that needs to be killed - teahouses generally have boiled water available. For all other types of fluid, iodine-base water purification tablets should work. I haven't found those yet, but have a small bottle of iodine that i'll take with if i can't find tablets.
Oh, and in my next life i'll be a travel doctor. You inject people with all kinds of things, and listen to their tales of things they have done, and what they are planning next. The only thing better than to hear travel stories all day, is collecting them yourselves!