Brain Rot is an alternate Pack-a-Punch ammo-type found in Call of Duty: Black Ops 4, and it has a chance to be applied to an upgraded weapon when placed in the Pack-A-Punch Machine again. It costs 2500 points initially, but lowers to 1500 after upgrading your weapon 4 times.
Brain Rot ammo has a chance to infect an enemy zombie and have them become the player's ally for about 10 seconds. When affected with Brain Rot, a zombie will display the words "Brain Rot" above its head and furiously hunt down regular zombies, instantly killing them. Even Zombie Tigers can become infected by Brain Rot, which can take down Marauders and Destroyers with a single lunge attack. After the 10 seconds, the affected zombie will explode, killing all other zombies around it. The kills obtained from this method count as Headshot kills, allowing one to earn points, XP, and performance camo progress at a faster rate. Brain Rot ammo is super effective against Fire Catalyst zombies, causing them to explode in a high-velocity blast.