Before You Go - consider the areas of Thailand you will be visiting and check for recent health alerts. The U.S. Center for Disease Control has an excellent site and the BBC site is another good resource.
Water - is not usually a problem in Thailand. Bottled water is cheap and available virtually everywhere except adventure venues. But Thailand is a tropical to sub-tropical region and may be extremely humid. Proper hydration is extremely important.
Food - Thai food is one of the rewards of a visit to Thailand. In restaurants and stalls frequented by Thais, it will be cheap, fresh and tasty. Common sense is the key. If something smells spoiled, don't eat it.
Avoid undercooked meats. One other caveat might be the seasoning. While some Thai food is quite mild, other dishes are downright firey. See some recommendations for Thai Food.
Hygiene - will be important for two reasons. First, Thailand is hot and in the dry season can be dusty. You won't perspire, you will sweat. Bacteria will will grow faster and and fungus infections are always an issue. Second, you will smell bad.
The Thais are fastidious people. You will seldom see a dirty Thai unless they are engaged in hot dirty work. As soon as practical they will bathe and change clothes. Bathe several times each day. Make your first stop after a tour or trek the shower. Then when you sit down for that cool drink, the Thais and your fellow travelers will appreciate your presence much more.
Toilets - especially up country, may be the squat type. And there will seldom be toilet paper available, even in some of the sit-down stalls. Toilet paper is available in most markets, so carrying your own is advisable.
Diseases - Prevention & Avoidance.
