Thailand is full of potentially dangerous viruses and bacteria that are easily avoided if you take the right precautions. You will need to be up to date with your vaccinations before you go, and should ask your doctor about which ones you need well before you leave. Ask for an International Health Certificate to be filled in. You should keep this together with your passport when you are abroad in case of emergency. Also, if you are taking prescribed medicine, remember to ask the doctor how much you should take with you, and don't forget to pack your prescription.
Malaria is a potentially fatal illness transmitted by the anopheles mosquito.
It is usually confined to the rural and forested areas of Thailand, and risk of exposure is highest at dusk when the mozzies come out. In the cities there is hardly any risk and you may not need to take the prescription anti-malarials if you know you're not going to a risky part of the country. If you do decide to err on the safe side they need to be taken daily, starting several weeks before you go to Thailand and continuing for several weeks after you get back. Your doctor should be able to provide you with a list of anything else, and may even sell you a medical travel bag quite cheaply. You will also need some sunblock, a pair of sunglasses and a hat.