The causes of the neurodegenerative disorders of Parkinson's disease (PD), Alzheimer's disease (AD) and amyotrophic lateral sclerosis (ALS) are unknown. It is proposed that all these disorders result primarily from a loss of trophic peptidergic neurotransmitter, possibly Substance P (SP). This loss in turn produces the classical neuronal degeneration seen in each of these diseases and occurs due to a combination of natural aging and chronic autoimmune destruction following a viral infection of the CNS, early in life. The loss is therefore slow and by the time of clinical presentation the inflammatory process is disappearing as the antigenic stimulus lessens with its removal. The implications of the theory in terms of future research and therapy are briefly discussed.