Poliomyelitis, often calle polio or infantile paralysis, is an acute viral infectious disease spread from person to person, primarily via the fecal-oral route. The term derives from the Greek polio (πολίός), meaning "grey", myelon (µυελός), referring to the "spinal cord", and -itis, which denotes inflammation. Although around 90% of polio infections have no symptoms at all, affected individuals can exhibit a range of symptoms if the virus enters the blood stream. In fewer than 1% of cases the virus enters the central nervous system, preferentially infecting and destroying motor neurons, leading to muscle weakness and acute flaccid paralysis. Different types of paralysis may occur, depending on the nerves involved. Spinal polio is the most common form, characterized by asymmetric paralysis that most often involves the legs. Bulbar polio leads to weakness of muscles innervated by cranial nerves. Bulbospinal polio is a combination of bulbar and spinal paralysis.