Blood in the Air is a symptom combo exclusive to the Necroa Virus DLC which causes zombies with heightened senses of smell to seek fresh blood. Severity increases when this combo is in play. Delirium and Hyperosmia are required. The Jaws achievement is earned for forming this combo.

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
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