Details of the event differ somewhat from those given in the movie "Serenity", due to certain inconsistencies and scientific questions left by the plot of the film. To summarize: much smaller colonist population, much larger % of 'em go Reaver, overall numbers of Reavers more or less the same. PAX introduced via the water supply (plausible) not the atmo processors (just silly).The "Miranda Event" was the disastrous loss of life following a failed chemical attempt at social engineering on the colony world of Miranda, and the subsequent failure of the coverup in 2518. The entire fiasco was ostensibly the product of a small, well hidden faction of Alliance researchers usually referred to as the "Miranda Project", though the project was signed off on by several heads of the Alliance ministries. Miranda was one of the last and most isolated worlds in the 'Verse to be terraformed and colonized, and settlement was done so in a more deliberately low-profile manner than most, to allow for experimentation. In 2506, the scientists of the Miranda Project deliberately introduced G-23 Paxilon Hydrochlorate or "Pax" to the civic water supply of the largest urban areas on Miranda. Pax had been covertly tested in the lab and the field, and found to be effective as an agent of social control. When deployed on Miranda, something went terribly wrong - the majority of civilians became apathetic to the point of death within days of exposure. A small minority became psychotic, later developing into "Reavers". A tiny fraction seemed entirely unaffected.
The agents of the Miranda Project went into action, creating a story about a "terraforming accident" that left Miranda uninhabitable - radiation from Burnham, radiation from the surface, virulent plagues... various carefully crafted rumors were allowed to flourish. News stories were suppressed, and travel to the system was discouraged. Because the trip is a long and difficult one, few attempted the voyage, and fewer still returned. The system was surrounded with navigation beacons warning off travelers and abandoned. In the decade that followed, the Unification War and the first appearances of the "Reavers" led to Miranda being largely forgotten by the public - a suppressed historical footnote.
In 2518, with the broadcast of the "Miranda Wave", this disaster, the deaths of millions at the hands of social engineers, burst into the open and shook the foundations of the Alliance.The Miranda Event caused widespread outrage and public protests, with extensive rioting on the Border Worlds. The Alliance Government attempted to quell public anger through trials and inquiries, and the execution of several convenient patsies. The Alliance buried the story amidst lots of other news and pensioned off a few senior members of the Alliance government. Numerous ideological groups were radicalized by the revelation of Miranda, and terrorism is growing more common on many worlds. In response the Alliance government has crack downed on its political opponents, intimidating critics, rounding up dissidents, and suppressing the media. Unfortunately, the damage is already done, and the Rim Worlds are growing restive. Here and there, small rebellions are breaking out. It is only a matter of time before they start to cooperate.
Alliance attention is currently focused in the Burnham Quadrant, attempting to root out the “Reaver Menace”. Miranda is under naval blockade, and much of the Alliance ground military is currently deployed in hunting the Reavers on the ground… something that the local Reavers find invigorating. Alliance naval presence is therefore spread much thinner than before, except around key Core Worlds. There is a great deal more smuggling, piracy, and anti-government space activity. Far more of the load is being carried by small patrol ships than before. After their defeat in the Battle of Dante following the "Miranda Event", the Reavers scattered across human space. They are now lurking and re-establishing themselves in empty quarters throughout the ‘Verse. Rumors of Reaver activity can be found across the 'Verse, and many privately owned vessels are beginning to arm themselves illegally. Now that the existence of Reavers is officially acknowledged, and incidents involving them are being tracked, useful statistics are being compiled. It appears that despite the destruction of a large number of Reavers at Dante, the number of known attacks across the 'Verse since then has not significantly diminished.