Nephritis is inflammation of the kidney. The word comes from "nephro" meaning "of the kidney" and "itis" meaning "inflammation". The two most common causes of nephritis are infection or an auto-immune process.
The disease is a problem because the inflamation has the effect of damaging and closing up the microscopic filters in the kidney. This means that in addition to various toxic waste products, the inflamed kidney filters out important proteins (larger molecules) from the blood. Therefore the characteristic symptom of nephritis is proteinuria - meaning the excessive removal of protein from the blood and its excretion in urine. Loss of blood protein(s) can result in a number of problems including swelling, excessive water retention, and the danger of blood clots (as a result of the removal of anti-coagulation proteins). It is a serious, incurable condition, but it goes into spontaneous remission in about half of all cases. It is treated with immunosuppresant drugs and cortiscosteroids. These drugs appear to increase the chances of speedy remission, but there is much medical debate about their effectiveness in this situation.
Gary Coleman is a famous sufferer of this ailment. So were Emily Dickinson and Gregor Mendel.