Ketone bodies (also known as acetone bodies) are any of the three compounds created by acetyl coenzyme A: acetoacetate, hydroxybutyrate, and acetone. These are water-soluble cellular fuels normally exported by the liver. They can build up in the blood and body tissues because of starvation (it can happen in extreme anorexia), untreated diabetes mellitus, or other disorders that interfere with carbohydrate metabolism.
The body rids itself of ketones mainly through urine, but it rids itself of acetone through the lungs, which gives the breath a characteristic fruity odor. If ketones build up in the body long enough, they cause a serious illness called ketoacidosis, which is characterized by nausea, abdominal pain, excessive urination, and lethargy. It can cause dangerously low blood pressure from dehydration, coma and death through cerebral edema or circulatory system collapse.
Treatment for ketoacidosis involves giving the patient intravenous fluids for the dehydration and carefully-monitored intravenous glucose or insulin, depending on whether the patient was diabetic or starving. Fluids are given by mouth whenever the patient can tolerate them.
Most of the information in this writeup was taken from the science dictionary at http://biotech.icmb.utexas.edu/; I oversaw the development of the dictionary (the website was mothballed in 1998) and I believe I wrote the entry this is based on.