Zombies are undead entities created and led by the Undermind, a zombie gestalt and a villain in the Ghostbusters comic book. They are essentially dead bodies without spirits.
The Undermind, a zombie gestalt, simply exists to expand and eat. It endures. During an incident, the Undermind spread its Infestation virus across another dimension. Those infected respond to the Undermind's autonomic appetite so it can prosper and survive. Things go from bad to worse, when a vampire named Britt is infected with the virus. She opens several interdimensional portals and spreads the virus to four iterations of the universe. Avatars of Britt coordinate the spread in each.
Late in the evening to early morning hours, a drunk driver hits one of the zombies. Upon investigating, the man is bitten by the zombie. Two days later, now nearly transformed, he appears at 20 Clinton Avenue to Peter Venkman and Ray Stantz. Egon Spengler concludes he has no soul left. The team continues to watch the hastened ectoplasmic migration from a corporeal form, the soul leaving the body. However, once the soul is gone, the zombie becomes aggressive and attacks. It absorbs the Proton Streams and implodes, releasing Necrotic Slime. Some of the slime gets on a stray cat. The team combines their streams and vaporizes "Fluffy."
The Firehouse is inundated with calls about zombies then one zombie enters the building. After Janine Melnitz throws a full Trap at it, the poltergeist inside possesses the zombie. The zombie acted as a biological containment grid and both were altered on a subatomic level. 15 minutes later, Egon realized they could remove the poltergeist and keep it in the Containment Unit indefinitely. While he went to rig up a method of tracking the zombies, a news report revealed the Stay Puft Marshmallow Man marching on West 57th Street. An avatar of Britt influenced all 37 zombies to stop randomly attacking and replicating and migrate towards Stay Puft. She planned on using them to infect Stay Puft and weaken Gozer so she could capture part of its essence.
The zombies infect Stay Puft on a massive scale so much he tears into two entities: a weakened Stay Puft and a Necrotic Stay Puft Marshmallow Man. The Ghostbusters release dozens of poltergeists who then possess the Necrotic one. The gamble pays off and it is destroying, effectively destroying all zombies infected with the Infestation virus. However, the Britt avatar decides to leave some Necrotic Slime in Newark, New Jersey.