Chernobyl is an origin story exclusive to the Necroa Virus DLC.
Mutation 1.5 introduced the Necroa Virus to Plague Inc., and with it many new achievements. Three of these achievements revolve around the disease originating in animals in the Chernobyl exclusion zone. Hide and Seek requires that special forces abort their mission after being unable to locate carrier animals. Test This! requires that tests on Chernobyl animal fail. Breathe Deep requires that the Chernobyl research lab be compromised.