From Sonic Retro
The Metal Virus is an object that was used in the Sonic the Hedgehog comic series by IDW Publishing. It is an artificial virus created by Dr. Eggman that was designed to transmutate organic matter into metallic matter, with the intention of increasing the size of Eggman's army.
The Metal Virus was the primary focus of what is considered the "Metal Virus saga", which lasted from issues #13 to #32 as well as the 2020 Annual. The virus had nearly destroyed Sonic's world, but every single trace of it was destroyed by Super Sonic and Super Silver in issue #29. Dr. Starline later repurposed the Metal Virus for Surge and Kit as safe cellular upgrades supplementing their cybernetic enhancements.
The Metal Virus was originally conceived for Archie Comics' Sonic the Hedgehog comic series. A previous editor for the comic asked Ian Flynn to include a zombie story in the Sonic comics due to the popularity of Afterlife with Archie and The Walking Dead. Flynn was initially hesitant to do it because he didn't feel it would work with Sonic, and by the time it would come out people would be tired of zombie stories. He was still told to make it work, and he eventually came up with an idea he liked, but the upper management of Archie didn't want it to steal the thunder of Afterlife with Archie so he had to shelve the idea. Ian Flynn didn't want to give up on the idea of the story though so he asked multiple times if he could do it.
In the original story, it would have replaced the original concept of Roboticization in the post-Super Genesis Wave reboot. Not much development beyond that was done at the time. Had the series not been cancelled, the Metal Virus would have been introduced post-issue #300.
After the first arc of the IDW Sonic comics was completed, Ian Flynn brought back his old Metal Virus idea and pitched it to IDW Publishing and Sega, to his surprise it was approved. He didn't get much push back at all on the story from either side and got to tell the story he wanted to tell.
The Metal Virus, when viewed at a microscopic level, appears in the form of nanobots that resemble six-legged bacteriophages bearing the Eggman Empire logo on their heads. To the naked eye, the virus has the appearance of a grey metallic liquid.
When the Metal Virus comes into contact with active organic tissue found on flora or fauna, it will begin to spread about and infect their cells, rewriting their genetic coding at a molecular level into a metal-like substance. It is contagious like an actual virus, as even the slightest touch is enough to spread the virus to others. The rate at which the virus spreads across the body depends on the situation. From a simple touch, the transmutation process is slow, but repeated exposure to the virus will accelerate the process, and the process can be painful. If a subject is exposed to full saturation of the virus, total transmutation will occur instantaneously. The virus cannot infect inorganic or processed matter, such as stone, metal, food or processed wood.
When transmutation is complete, the subject's entire body becomes entirely composed of the Metal Virus, becoming metallic. Transmutated subjects' bodies become much more durable and acquire the ability to reconstitute themselves if they are damaged, making them virtually indestructible. Fauna that is fully converted by the virus are transformed into soulless, zombie-like robots that are subservient to Eggman's orders, aptly named "Zombots" by Sonic the Hedgehog.
Like real-life viruses, the Metal Virus is capable of evolving through mutations in its coding. Through constant exposure, the virus had mutated in that Zombots gradually became more disobedient of Eggman's orders, requiring the use of override frequencies or electromagnetic energy to control them. There is no known vaccine to the Metal Virus, but it was discovered that Sonic's super speed can counteract the virus by driving it into remission, but not outright eradicate it. However, the virus evolved further, causing Sonic's speed to slowly lose the ability to reverse its effects.
Through research, Eggman eventually reveals that as the Metal Virus mutates, it becomes unsustainable. Any infected heterotrophic eukaryote cells will eventually suffer a form of apoptosis in about two hundred years time; autotrophic cells will endure slightly longer, but not by much. This means that the Zombots will eventually disintegrate, therefore by the time Silver's future happens, all fauna is gone, with only sparse metallic plant life still remaining.