Pythium insidiosum is a protoctista and causes diseases in plants and animals. In Thailand it can cause a unique human infection of three types. The first type is a subcutaneous lesion in thalassaemic patients, with the pathological findings of a granulomatous reaction, diffuse infiltration, and oedema of the vessel walls. The patients responded to a saturated solution of potassium iodide. The second type is chronic inflammation and occlusion of blood vessels mainly in the lower extremities which results in gangrene or aneurysm formation. This type of infection is only found in thalassaemic patients and leads to amputation of the affected extremities or resection of the involved arteries. The third type is keratitis. This type of infection may or may not be associated with thalassaemia. The clinical signs and symptoms do not differentiate it from other types of myocotic keratitis. The patients end up with keratoplasty, evisceration or enucleation. Thailand is an agricultural country, and there are plenty of swampy areas and several plants to support the life cycle of Pythium. Moreover, many people suffer from thalassaemia, and there is no drug available for Pythium. Pythiosis plays an important role in medicine in Thailand.