Although many HIV patients have a tendancy to develop anemia (low blood count), it is indeed true that some develop too high of a red blood cell count. This condition goes under various names including erythrocytosis and polycythemia. His doctor gave him the correct advice in that patients with too high a red cell count have an increased risk of stroke. There are many different treatments for this condition. A very common treatment for this condition is the removal of a unit of blood every so often (could be once a week, once a month, or once every few months depending on the patient). It's important that if you brother is on drugs which could cause this, then he should stop them. Testosterone derivatives and steroids, which are taken by many HIV patients, can cause this condition of too many red blood cells.
