Plague 5

Plague 5, named for the city it first arose in (Mira 5 in Sector 71 on Mars) was an illness that swept over half the planet and caused a death toll in the millions. For reasons that we have yet to explain, the P5 was unable to survive aboard starships, and thus the outbreak was contained to the surface of Mars. Even so, the delicate balance of life on Mars was thrown into chaos for a time.
P5 manifests in the form of an odd rash. It travels from shoulder blades, down the arms and envelops the hands. As the illness continues, if untreated, it will cause one's body to rot in every place touched by the rash. If this rot enters the bloodstream, it is often deadly. Treatment and a cure were discovered five months into the plague, but societal factors meant actually getting these things to those in need took far too long. 
At the time (year 3145), Mars was still a place for lower-class workers to farm and produce goods for those who owned the companies they belonged to. Most of these companies took the stance that it would be far less expensive to simply replace their workforce instead of paying for and shipping in expensive treatments. 
A large amount of pirates did business with those on Mars during this time, a relationship that allowed the pirates to sell the goods they stole for the profit they were seeking and gave those on Mars supplies that their companies wouldn't purchase for them for one reason or another. As this tragedy progressed, this symbiotic and illegal relationship grew. Many pirates demanded higher prices for the goods they were able to acquire, and out of desperation, the Martians paid. There were, however, some pirates who managed to get their hands on the information on how to produce the treatments for P5 and delivered it to several Mars-based scientists, seemingly for free. The names of these pirates will never be known, but they are considered heroes.
With this knowledge, the Martians were able to complete their own research and craft treatments needed for those who were sick. A strain of P5 still exists, but largely it is no longer an issue in the current year. The effects of Plague 5 were far more widespread than the sickness itself. The Martian Strikes of 3147 were what many referred to as the true tipping point toward true Martian independence, but those strikes would not have happened if the companies that owned Mars had acted differently during Plague 5.

Origin

Natural