Check with your doctor for the latest prophylactic drugs, although many doctors consider it best not to take anything. The test for malaria is very quick and reliable, and a course of drugs stops the disease effectively. The most common problem is diarrhea. This usually only lasts one or two days, and is generally a reaction to the change of climate and diet. Immodium or a similar medicine is fast acting and effective, and quite safe to take for a day or two.
Dehydration can be a problem in the heat.
Oral re-hydration solution is excellent at replacing lost electrolytes, and can be bought at any pharmacist. However, this can easily be avoided by drinking plenty of water. Sun tan oil is also often necessary to prevent sunburn, which can easily spoil a holiday. Photography Thailand is a paradise for photographers, so be sure to bring a camera and film. Film and processing are cheap, very fast and the quality usually very good.