Paralytic poliomyelitis occurs in less than 1% of poliovirus infections. Oral polio vaccine contains live attenuated (weakened) strains of the three serotypes of poliovirus. Polio, short for poliomyelitis, is an infectious disease caused by the poliovirus. Other causes of acute viral encephalitis are rabies virus, poliovirus, and measles virus. Historically, poliomyelitis was the most significant disease caused by an enterovirus, namely poliovirus. Poliomyelitis, caused by poliovirus often occurred in the summer months. Two additional challenges are found in unobserved polio transmission and in vaccine-derived poliovirus. The most important step in eradication of polio is interruption of endemic transmission of poliovirus. Cases of circulating vaccine-derived poliovirus type 2 continue to appear in several countries. Examples include the poliovirus, Hepatitis C virus, and Foot-and-mouth disease virus. There were 32 circulating vaccine-derived poliovirus (cVDPV) cases in 2015. The molecular mechanisms by which poliovirus causes paralytic disease are poorly understood. Drake demonstrated that poliovirus is able to undergo multiplicity reactivation. This type of poliovirus was declared eradicated. Infections by poliovirus most often cause the mildest of symptoms.