Plague Snails are creatures that live in the Eternal City. They are the source of the City's virus, and are collected by the Scientists for creating the Styx's viruses. They possibly emit the viruses through their membrane/squishy part and put the germs into the City's air. In Closer, Drake and Eddie form a plan to destroy the Snails and end the Styx's plans of making viruses.
In the series Edit
While Eddie is showing Drake his collection of things in his warehouse hideout, Drake notices some bottles containing what appear to be dead snails. Eddie says that they are Plague Snails, the source of the virus in the Eternal City. After explaining how the Styx use them to create their viruses, Drake forms a plan to destroy the Snails to thwart the Styx. Eddie says it will be hard (there are many snails and the Eternal City is a big place), but with help from a pesticides expert, the plan slowly forms. Drake and Eddie pack for the mission into the City, containing pesticide bombs they will plant at intervals. Chester tags along with them as they enter the underground city and knock out a Styx Division patrol. They then plant the bombs and detonate them, releasing the gas and killing the Snails.
Near the end of the book, the Old Styx's young assistant tells his boss that Drake had wiped out all the virus in the Eternal City, meaning all the Snails were killed and the mission was a success.
Other stuff Edit
Though not mentioned directly, it can be guessed that there are different types of Plague Snails. The Styx have been collecting specimens for decades (Drake says the Styx released viruses as far back as the 1918 Spanish Flu). Since the Spanish Flu, Asian and Eastern Flus, and Dominion are different Styx viruses, there is variety in the Snails.