From Sonic Retro
The Metal Virus is an object that is used in the Sonic the Hedgehog comic series by IDW Publishing. It is an artificial virus created by Dr. Eggman that is designed to transmutate organic matter into metallic matter, with the intention of increasing the size of Eggman's army.
According to writer Ian Flynn, the Metal Virus was planned for long in advance before the start of the IDW series. He later revealed that the Metal Virus was originally conceived for Archie Comics' Sonic the Hedgehog comic series to replace the original concept of Roboticization in the post-Super Genesis Wave reboot. Had the series not been cancelled, the Metal Virus would have been introduced in issue #300.
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. 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. Their 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.