Poliomyelitis, often called 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 poliós (πολιός), meaning "grey", myelós (µυελός), referring to the “spinal cord”.  Although around 90% of polio infections cause no symptoms at all, affected individuals can exhibit a range of symptoms if the virus enters the blood stream. Poliomyelitis was first recognized as a distinct condition by Jacob Heine in 1840.  Its causative agent, polivouius, was identified in 1908 by Karl Landsteiner.  Although major polio epidemics were unknown before the late 19th century, polio was one of the most dreaded childhood diseases of the 20th century.