If you are travelling to Thailand then you should check current advice for immunisation and injections BEFORE travelling. Of course many travellers and long-stay visitors do not take any injections or medication but it is always a good idea to be fully informed so that you can make a good decision.
Be aware that all people including visitors and locals are vulnerable to tummy upsets and diarrhoea. I have worked with many Thai people and upset stomachs (ท้องเสีย – tong see-a) are absolutely commonplace. Of course, this depends on where they eat.
In my uninformed opinion with no medical training it is always a good idea to carry some rehydration salts (น้ำเกลือแร่ – nam glua rae) which are very cheap to buy and can be found at any Thai pharmacy or convenience store including 7-Eleven and Family Mart. However, these are not a substitute for obtaining professional medical treatment which is usually cheap and readily available.
Guidebooks and medical staff from western countries tend to advise against use of un-bottled water ice in drinks but I have never knowingly found this to be a problem. On the other hand, I would agree that fresh cut fruit can be a problem.
More travel advice can be found on the NHS website.
Lastly, as one wise farang once quipped, “Never trust a fart in Thailand.”