Lotta Latte's spring water was swapped for Liquid Chicken Debrainer
The Liquid Chicken Debrainer is generally water combined with chemical subjects. This substance is usually used by McFingers.
If someone drinks it while it's cold, its effects are completely neutralized. However, if someone drinks it while it's hot, it turns the drinker into a zombie.
In The Brewed, a deliverer made a mistake by accidentally delivering the debrainer to Lotta Latte and the spring water to McFingers, causing half of Whitechapel to be transformed into zombies.