The harsh climate and the high altitude aren't  favorable environment for bacteria and viruses, so you don't need worry too much about getting disease while you travel in Tibet.

The possibility to catch this diseases is quite low in Tibet, easily avoidable by following simple rules: 

* wash hands before each meal (preferably with soap)

* consider all water to be infected, only boiled, bottled or sterilized water is safe!

* immediately disinfect any wounds

* all food should be cooked well, avoid re-heated food

* fruits and vegetables should be peeled before eaten

* try to eat off clean utensils, consider to carry your own utensils

* drink plenty tea or other boiled/canned liquids

Since boiling temperature is somewhat lower in Tibet, it is better to boil water for longer period of time.