Parkinson's disease is an advanced nervous system disorder that affects movement. Symptoms start slowly, sometimes starting with a hardly noticeable tremor in just one hand. Tremors are common, but the disorder also usually causes stiffness or slowing of movement. Parkinson's disease signs and symptoms can be unlike for everyone. Symptoms frequently begin on one side of your body and usually remain worse on that side, even after symptoms begin to affect both sides. Different people experience progress at different speeds, as well. Still, physicians have established stages that define how the disease progresses. These five stages of Parkinson's are known as the Hoehn and Yahr Scale used by physicians during the world to classify patients in research studies.