It’s usually caused by damage to the central nervous system especially the spinal cord. Causes include stroke, trauma, nerve damage, polio, multiple sclerosis, etc. Paralysis can occur either partially or fully. Lower half paralysis is called paraplegia and paralysis of both arms and legs is called quadriplegia. It happens because the brain can’t send messages to the different parts of the body for various reasons. If the nerve fibres are damaged the paralysis is usually irreversible.