 Two additional challenges are found in unobserved polio transmission and in vaccine-derived poliovirus.First, most polio infections are asymptomatic or result in minor symptoms, with less than 1% of infections leading to paralysis and most infected people are unaware that they carry the disease, allowing polio to spread widely before cases are seen.In 2000 using new screening techniques for the molecular characterization of outbreak viral strains, it was discovered that some of the outbreaks taking place were actually caused by circulating vaccine-derived poliovirus, following mutations or recombinations in the attenuated strain used for the oral polio vaccine.