Vegetative state is a disorder of consciousness that occurs when there is a dysfunction of the cerebral hemispheres caused by a traumatic brain injury, stroke or an illness. When this occurs, the person becomes unaware of his environment, but since the hypothalamus and the brain stem is not affected, the person’s autonomic and motor reflexes such as sleep cycle, blood pressure, heart rate, etc, still functions properly. The person is said to be awake but unaware.
Because the brain stem is what transfers all information to the rest of your body. Also, it is directly connected to the hypothalamus, which if damaged, can take away your ability to breathe or do simple tasks.