For this challenge, you must clear out 15 infestations for the reward of a cool liveried sports car.
Infestations are usually in houses but can be at camp sites and fields etc, and are easily spotted on your map by the big red blood splash around the area where an they are.
To clear them out you have to Kill around 10-15 zombies that come from the infested area, as well as an alpha or 2, usually screamers, and ferals, with the occasional bloater. (don't worry, no juggernauts here)
Easy tactics is to get a car and sound the horn around the area and just let them run out towards you for the splatter, or for more of a challenge get stuck in with some CQC. (2x4 not recommended)
Once cleared you get a notification on screen and the Blood splash disappears from the map.
to complete the challenge, do this 15 times before the end of may (2015) and a sweet unique sports car is yours, which can be collected from the radio menu in game once you have a base set up at the cost of 250 influence.