Touristas or diarrhea is caused by a tiny organism that lives in water and does not die if frozen. This organism can be different every 100 miles of travel. So do not drink the tap water and do not eat anything with shaved ice. If the ice does not have a hole through the middle don’t drink or eat it. Shaved ice comes from a block of city water. The block of ice is transported to a shaver facility often on the back of a motorcycle in the open BKK air where it dropped on the concrete floor where the employees and transport walk on the floor in their outside shoes thus BKK’s dirt is transferred to the floor from the bottom of the shoes then it is on the ice as it is lifted from the floor to the shaver machine and bagged and motorcycle transported back through the BKK air, rain and road splash and all.
Thai’s have a very different definition of clean from Caucasian westerners right down to washing hands and washing ingredients before preparing them and cooking them. Cleaning of pots and pans and business inspection is never what would be required for a North American restaurant. Street food is just plain risky for a foreigner.
Touristas is different from food poisoning in that tourists symptoms include lower intestinal tract cramps 3 to 4 hrs after ingestion and usually, within 30 minutes of the cramps, you have diarrhea. Tourists are treatable with Immodium (or its generic loperamide hydrochloride). Start treatment at the first lower intestinal cramp and that may prevent diarrhea and your day will not necessarily be ruined. You should also include a regime of activated charcoal following your Immodium as the Immodium stops the cramps and diarrhea while the charcoal gathers up the bug and removes it from your system. I lived here now for 10 years and never, never, leave home without Immodium.
On the other hand, may start with lower intestinal cramps then diarrhea but quickly advances to stomach convulsions (puking right down to bile) and includes hot and cold flashes and sweats. You are physically and mentally in trouble. Food poisoning is best treated by a hospital and can kill so you need to take it seriously and seek medical treatment.
Eating at expensive highly reputable restaurants is no guarantee you will not get food poisoning. Part of the reason is Thai culture 101– Thai culture is non-confrontational and that means if Thais are poisoned they do not tell the restaurant and seek retribution but rather they tell everyone they know and hopefully that reduces the restaurant’s business and the loss of customers effects a change or closure. But in reality, it means the restaurant continues to poison its patrons as Thai’s do not cultivate repeat customers.