Part of what makes Marvel's symbiotes such an insidious threat is that they can reform and reproduce from almost nothing. So long as a part of them exists, they are never truly dead, only dormant for an uncertain amount of time. But there is more to these creatures than just being living weapons, as they can also be utilized as a form of disease. And in Brian Michael Bendis and Mark Bagley's Mighty Avengers #7, one villain's attempt to frame Doctor Doom for the use of a biological weapon consumed New York and nearly unleashed a plague that was almost as bad as that seen in Marvel Zombies.
Kristoff Vernard, who Doctor Doom took in and trained as his protege at a young age, was determined to get his surrogate father arrested. Knowing that the tyrant would never be tried and prosecuted in his native Latveria, Kristoff attempted to force the hand of the international community in one of the worse ways imaginable: by creating and unleashing a virus based on the Venom symbiote upon one of the most populated places on Earth.
Using time-travel, Kristoff acquired a part of the symbiote during the time it was held captive by the Fantastic Four. From there he engineered a virus based on the symbiote. While it was not truly alive, it could infect its hosts in a manner similar to that of any ordinary symbiote and was capable of reproducing at a much faster rate. Kristoff then had the virus sold to Doom through proxies so his crime could not be traced back to him. Knowing the danger that such a virus could entail, Doom isolated it on a satellite orbiting the Earth, which gave Kristoff his opening to strike.
Kristoff then had the satellite deploy the virus, making it look like an accident. The weapon landed in Manhattan, where it proceeded to infect thousands of individuals, including many of Earth's heroes. The plague spread throughout the city, taking over individuals like Black Widow, Iron Fist, and Doctor Strange. Even heroes who had experience dealing with symbiotes in the past, such as Spider-Man, fell to the virus.
The result was an army of animalistic predators who successfully infected over 12,000 people. Fortunately, Iron Man managed to synthesize a cure that restored the population to normal. He eventually became the guardian of the virus, keeping it under lock and key to prevent any further catastrophes.
The Venom Virus has many similarities to the plague depicted in Marvel Zombies. Much like on that Earth, the Venom Virus spread rapidly, overwhelming and transforming various heroes into killing machines that turned on friends and allies. Similarly, both viruses were able to spread in a very short amount of time.
This begs the question of how prepared the heroes of the Marvel Universe are for a threat of this magnitude. The parallels to how close they came to another plague universe are too many to ignore. A disease nearly overtook the world, it turned heroes into monsters, and had they not been able to get a handle on it when they did, it could have easily created a world very much like what was seen in Marvel Zombies.