The polio virus which causes poliomyelitis in humans is an enterovirus which belongs to the picornavirus (small, RNA) family. Polio virus is rapid, acid-resistant, stable, highly tissue specific and consists of a single-stranded, positive RNA. Polio virus is able to reside in the throat or intestinal tract of humans. Poliomyelitis is a highly contagious infectious disease which has three strains, poliovirus 1 (PV1), PV2 and PV3. Polio virus, although rare in developed countries, can be found in many under-developed countries due to the uncommonness of vaccinations there. Polio is known as a disease of development. The oldest known record of polio is in an Egyptian stone engraving of a young priest from 1350 B.C. with a withered leg, characteristic of a polio survivor. Loeffler and Frosch were the first individuals to see polio in 1898. The largest US epidemic was in 1916 in New York City.