Hereditary hemorrhagic telangiectasia, also known as Osler-Weber-Rendu syndrome is a rare genetic disease where the body has difficulty building and supplying capillaries with blood. Instead, the body grows blood vessels directly between arteries and veins, bypassing the capillaries entirely and starving many of the body's tissues of oxygenated blood. The physical result of this is frequent nosebleeds from the malformation of blood vessels in the nose, and fatigue due to the lack of oxygen in muscles. As the disease progresses, capillaries in the lungs and heart can be bypassed, eventually leading to death of those organs.
The disease is not easy to diagnose as it is very rare and many of the symptoms are common to other diseases - a classic zebra diagnosis. A medical history is very important as the symptoms will be seen in male ancestors as well. It can also be diagnosed by the high level of oxygen in the veins.
However, once the disease is diagnosed, it can be treated by surgically blocking the malformed blood vessels. This restores normal blood flow to the capillaries. The surgery is so safe there is not a single case where a patient has died of complications following the surgery.