Successful eradication of infectious diseases has been achieved twice before, with smallpox and bovine rinderpest. Prevention of disease spread is accomplished by vaccination. Of the three strains of polio virus, the last recorded wild case caused by type 2 (WPV2) was in 1999, and WPV2 was declared eradicated in 2015. Of the three strains of polio virus, the last recorded wild case caused by type 2 (WPV2) was in 1999, and WPV2 was declared eradicated in 2015. Type 3 (WPV3) is last known to have caused polio in 2012, and was declared eradicated in 2019. All wild-virus cases since that date have been due to type 1 (WPV1).