The so-called Hate Plague was a spore-based contagion which nearly tore the known galaxy apart. Appearing as glowing red spores, the "plague" induced violence and hatred in its victims, causing them to fight and destroy one another. The spores could infect any intelligent being, from rats to humans to Transformers and Sharkticons.
The Hate Plague first appeared in the early days of Cybertron (Seibertron), but a being of incredible wisdom was able to resist the spores, imprisoning them within a star. Millions of years later, in 2006 (2010), the star went nova. In the explosion, the spores covered an Earth spacecraft, the Solaris (having just rescued the lifeless body of Optimus Prime (Convoy)), which brought them back to Earth.
The inventors of the spacecraft- Dr. Gregory Swafford, Mark Morgan and Jessica Morgan- discovered the nature of the spores, just before the Decepticons (Destrons) attacked to steal the heat-resistant alloy used to build the Solaris. In the process, Jessica was paralyzed from the waist down, leading Swafford and Morgan (who had his own grudge against the Transformers) to concoct a plan of revenge- they would rebuild Optimus Prime, infect him with the spores, and use him to destroy the Tranformers. However, while they were unable to revive Prime, they were able to infect a squad of Autobots (Cybertrons).
Unfortunately, Swafford and Morgan severely underestimated the spores, and wound up infecting not only most life on the Earth, but many beings on other worlds as well. To save the universe, [[Sky Lynx]s] (acting on the last uninfected orders of Rodimus Prime (Rodimus Convoy)) found an uninfected Quintesson, and had him rebuild Optimus Prime. Prime sought out the heat-resistant alloy on Charr (as it was the only material known to protect against the spores), and covering himself with it, he took the Matrix from Rodimus Prime. Unable to find a cure for the plague within the vast wisdom of the Matrix, Prime reasoned that perhaps the accumulated knowledge of the Matrix could purge the spores. He was correct, and the Hate Plague was wiped out, saving the galaxy.
Although the spores theoretically made their victims fight everyone, it seemed to turn the infected's wrath primarily against the uninfected. This was likely the spores' way of propagating themselves.