Alzheimer's disease has proven to be a difficult enemy to defeat. After all, aging is the No. 1 risk factor for the disorder, and there's no stopping that. Most researchers believe the disease is caused by one of two proteins, one called tau, the other beta-amyloid. As we age, most scientists say, these proteins either disrupt signaling between neurons or simply kill them. Now, a new UCLA study suggests a third possible cause: iron accumulation.