Nigeria
Nigeria is one of the most entrenched reservoirs of wild poliovirus in the world. It is the only country with ongoing transmission of all three serotypes: wild poliovirus type 1, wild poliovirus type 3, and circulating vaccine-derived poliovirus type 2.
States in the north of the country are the main source of polio infections elsewhere in Nigeria and in neighbouring countries. In 2009, operational improvements in these northern states led to a 90% decline in cases of wild poliovirus type 1 and a 50% decline in overall cases compared with 2008.