I have an extensive page on genetic diseases in purebred cats. But on this page I would like to refer to page 91 of The Cat, Its Behavior, Nutrition and Health where it mentions that the Siamese cat as a cat breed, has the following "associated disorders":
- Hyperesthesia syndrome - rolling skin disease and possible seizures and self mutilation.
- Intestinal adenocarcinomas: mammary tumors - forms of cancer, unusually high in Siamese.
- Hydrocephalus - excessive spinal fluid accumulation - autosomal recessive gene causes it.
- Congenital heart defects - more common in Siamese than other breeds.
See more at Siamese Cat Health Problems.