Malaria is a parasite spread by Anopheles mosquitoes. Many of the popular jungle trekking areas of Thailand, and a number of the rural islands present high risk to the travelers. Bangkok and most popular tourist centers and resorts in Thailand are considered relatively risk free.
However, the best prevention is simply using a good mosquito repellent, containing DEET avoid perfumes and scented after-shaves, and wear light colored long-sleeved shirts, long pants, and shoes. At night, sleep with air conditioning or under a ceiling fan; mosquitoes don’t like them. Symptoms include fever, chills, sweating, severe headache, and abdominal pains. Untreated Malaria can be fatal; treatment is, however, very effective and recovery is rapid, so if you have these symptoms, see a physician right away.