The three serotypes of poliovirus, PV-1, PV-2, and PV-3, each have a slightly different capsid protein. Capsid proteins define cellular receptor specificity and virus antigenicity. "PV-1" is the most common form encountered in nature, but all three forms are extremely infectious. As of March 2020, wild PV-1 is highly localized to regions in Pakistan and Afghanistan. Wild PV-2 was declared eradicated in September 2015 after last being detected in 1999, whilst wild PV-3 was declared eradicated in 2019 after last being detected in 2012.