Feline Leukemia

• The feline leukemia virus impairs a cat's immune system. It can cause cancer in cats, as well, according to PetPlace.com.
  
  

Feline AIDS

• Feline immunodeficiency virus, or feline AIDS, can cause cancer, anemia, or infection due to the cat's weakened immune system. There is no known cure, but a cat's symptoms can be treated with medication or surgery.
  
  

Feline Idiopathic Cystitis

• Feline Idiopathic Cystitis, also known as FIC, is an inflammation of a cat's urinary bladder. Symptoms to watch for include distress while urinating and increased grooming of the genital area, according to PetPlace.
  
  

Hypertrophic Cardiomyopathy

• This disease is characterized by a thickening of the left ventricle. It can cause blood clots that impair blood flow in the body, and severe cases can cause heart failure.
  
  

Diabetes Mellitus

• Diabetes mellitus is a chronic deficiency of insulin in cats. The disease, as with diabetes in humans, can be treated with daily insulin injections.