Plague Inc. Wiki

Boomer is a symptom combo exclusive to the Necroa Virus DLC which allows zombies to explode when excited. Severity increases when this combo is in play. The symptom Autothysis and the ability Liquefactive Necrosis are required. The achievement Boomer is earned for forming the combo.

It may based off of a type of zombie from Valve's Left 4 Dead franchise thats explodes when player kills and vomit biles to attract zombies to attack them.

Symptom Combos in Plague Inc. and Plague Inc: Evolved.
AssassinBath TimeBlind GeniusBlood in the AirBoomerCarpe JugulumCranial DispersionDiscovery ChannelFly MagnetOn The Naughty ListOopsProfuse BleedingProjectile VomitingPublic DefecationRed Ape RedemptionRed RainRude AwakeningRunnerSpitterSwine FluTankUltimate ChristmasVampire BatWalking ContradictionWalking DeadZombie Panic