When Megatron conquered Cybertron, his most potent weapon was a virus that subdued the population almost overnight. It had many effects, though the primary intended ones seemed to be mode-lock and deactivation. It has also been associated with paralysis, short-term amnesia, and "devolution."
|“||Nobody saw it coming: a virus suddenly spreading over the whole planet. Everyone was infected. Many deactivated; most just disappeared.||”|
At some point before unleashing the virus on Cybertron, Megatron tested it on a group of Fuzors. But rather than freeze their transformations and shut them down, it somehow gave them two separate beast modes at the expense of their robot modes. They also gained special powers like telepathy and teleportation. Unsurprisingly, they were able to escape, and they became the renegades known as the Mutants.
Megatron presumably improved the virus after that incident, since the Fuzor Silverbolt would later be one of the few Beast Warriors who succumbed immediately and completely. He also showed no signs of spontaneous superpowers.
As recounted by Nightscream, Megatron's viral assault came massively and without warning. It was actually visible as a pillar of green... something erupting in the center of a city, then falling back down and sweeping through the streets as a rapidly-moving cloud. Those who weren't immediately deactivated were hunted down by the ever-growing hordes of Vehicons. Despite "everyone" being infected (presumably including himself), Nightscream managed to stay mobile even as the Vehicon ranks increased into the thousands. Running away from an attack, he fell down a deep pit and "went offline." His internal DNA scanner discovered a fossilized bat and used it to give him a beast mode. When he reactivated, he had no memory of the bat or the chamber it was in, and he was stuck in his new beast mode. However, he also found an organic tree with edible fruit, which he used to survive for some time. He would later credit his beast mode with his resistance to the virus. However, the fruit diet may also have been a factor, as he told Optimus Primal and his reformatted Maximals, "This'll make you better."
The Maximals themselves had also been struck with amnesia and mode-lock upon their exposure to the virus, though that wasn't the only effect. After their ship crashed, they were hounded by Vehicons firing explosive canisters. The canisters released a glowing, green gas in an apparently much smaller-scale version of the original planet-wide viral release. All of the Transmetal Maximals were immediately "devolved" into their original beast modes, though they were still mobile. The non- and partially-Transmetallized, however, remained in their robot modes, seemingly paralyzed as the Vehicons overtook them.
While the de-Transmetallized Maximals did evade capture, they lost their memories of the entire event, forgetting even Megatron's escape from their shuttle in transwarp. They were trapped in beast mode and slowly shutting down, electric jolts occasionally sparking from their bodies. Primal and Blackarachnia both somehow concluded, independently of each other, that they could avoid shutdown if only they could transform. But without that option, Primal followed visions to the Oracle, at which point the Maximals seemed on the verge of deactivation. However, the Oracle reformatted them into technorganic bodies instantly energized and immune to the virus.
The Maximals eventually found the laboratory where Megatron had manufactured the virus. Rattrap, frustrated that he couldn't get his new body to transform out of beast mode, correctly anticipated that there would be a "counter-virus" with the opposite effect. Taking it, he found himself transforming uncontrollably.
With Maximals' technorganic immunity making the virus practically irrelevant, little more was learned about it or its relationship to the green gas. It made one more possible return when Botanica arrived from offworld, though the evidence is scant. Her ship was shot down, and the investigating Maximals found themselves attacked by her feral, seed-scattering plant-mode. She was drawn underground to the technorganic orchard and its pipeline to the planet's organic core. Once there, she began to spark like the Maximals had when they were infected, giving Primal the idea to reformat her. The reformatting successfully restored her mental and bodily health, and she said that Megatron's attack on her ship had "shorted out" her "higher reasoning protocols." She had been running on pure self-preservation, lashing out at anything, but the reformatting had "rebooted [her] primary operating systems." With that explanation, the only viral symptom in evidence was the brief sparking. But whether or not Primal was right in his diagnosis, this would be the last mention of the virus.
Watching in astonishment at Megatron's takeover, Apelinq was able to protect at least himself by creating a shield suit against the "residual effects" of the plague. Another advance came when he reunited with Rodimus, whose "unique physiology" had made him immune to the virus. Apelinq created an inoculant "by mapping and replicating his Spark's low-band energy signatures." When Primal Prime and his Maximals arrived later, Apelinq intended to inoculate them. But, according to Primal, Apelinq was attacked and hurled through a transwarp gateway "before he could inoculate the rest of my crew." What that phrase implies for the Wreckers' immunity or lack thereof is open for speculation.
The Beast Machines summary in Transformers: The Ultimate Guide claims that the technorganic Maximals' transformation difficulties are the result of the "last vestiges" of the virus. This is somewhat at odds with Optimus Primal's dialogue, which places the blame on their new bodies' lack of onboard computers. However, if the virus is at least part of the cause, this could help account for the fact that all of the Maximals reformatted later are able to transform instantly. Nightscream, while most likely infected, showed markedly light symptoms all along (and he had already watched Primal and co. practice transforming for some time). Botanica showed even fewer symptoms before her reformatting, if she was infected at all. And Silverbolt was reformatted from his almost-certainly-immune Vehicon body.
Involved in the ambiguous nature of the virus is its connection to the mysterious green gas. Its first appearance is during Nightscream's flashback, where it appears synonymous with the virus itself. However, if this "virus" is like any other known virus, it shouldn't be visible. This leads to the notion that the gas is perhaps a delivery system. But if Botanica was infected on her arrival despite the gas being absent, then the gas isn't necessary for the virus to spread.
Additionally, the "devolution" of the Transmetals occurred when they were exposed to the gas. This has led some fans to think that the gas devolved them, not the virus at all. However, while the non-Transmetals seemed to be instantly paralyzed, the de-Transmetallized were able to keep moving. So other fans have suggested that Transmetallization was an unforeseen buffer against the virus's full effects, the de-Transmetallization process buying them time before shutdown.
Ultimately, though, the "true" nature of the virus and gas are unknowable, as the fiction contains no clear explanation.