It's all a matter of concentration. The human body is able to mobilize it's natural defenses when bacteria is introduced into the body. Small amounts of bacteria (usually e-coli) are engulfed and pass right on through. Residual tap water in a washed beer glass or a teaspoon consumed while brushing teeth present little problem for a healthy person. Gulping a glass of foul tap water in the middle of the night is a different story. Additionally, water does not allow bacteria to colonize (nothing to feed off of), so unless you run across some exceptionally bad stuff, you have little to worry about.
I think conventional wisdom leads people to believe that it was water that made them sick, when actually it was more likely something they ate. Bacteria can multiply at an astronomical rate given the right medium. A warm pan sitting around for 5 or 6 hours is quite conducive to bacteria growth. Couple that with the overpowering nature of Thai food, spicy, often salty and sour, (Curry came about long before the invention of the refrigerator to disguise the taste of spoilt foods) and the unsuspecting Farang ingests a meal of healthy bacteria in the prime of their short lives, hell bent on reproducing with abandon anywhere in a warm digestive tract.
The onset of symptoms can be abrupt. Eat a meal at 0200, back to the hotel, sleep, then up at 0500 for an urgent visit to the loo. The body mobilizes it's defenses, but cannot cope with the invasion. Fever begins. Too weak to even leave the bathroom you embrace the cool porcelain until the wretching subsides. Forget about going to a pharmacy for medication, you cannot even stand up-right...besides, nothing will stay down.
Eventually, your body purges the bacteria from both ends, and regains control. You'll feel better within 18 hours. At this point you'll desperately crave some kind of electrolyte solution. The thought of beer makes you gag. Bet you wish you had some gatorade in your mini-bar!
Sorry, I got carried away. My point: Prepare for the worst, but don't let it consume your holiday. Be reasonable about where you eat. I've had some great meals from street vendors, but also got the above malady from street food. If you aren't "local", don't eat like you have been living in Thailand your whole life. Have some familiar medication with you (if you need it, you'll be glad you have it), and ALWAYS keep, at the very least, a few litres of bottled water in your mini bar. At least this way if intestinal disaster strikes, you'll be prepared.