Virulent, utterly destructive and utterly horrific in its capacity of wiping out the population of Colony. It didn't kill with just toxicology and the physical aspects of the body, but it took three days to show symptoms, three days where it infected the lungs, driving deeper and deeper until the deepest parts were afflicted, and then it would begin to terminate. From start to finish, it was virulent, anyone who breathed the same air as the infected would be infected, and would spread the disease not an hour later.
Two colonial Habitats had to be destroyed, the machinery that kept the colony in the "safe" zone of the planet had been deemed not worth the risk of infection.
It was thankfully kept on the planet due to strict screening procedures, and none of the military crews were affected bar the Dreadnaught. It was kept in orbit, it's atmospherics system was used to pump a burning agent to clear the ship of the virulent from the vents. It took two months for it to be recovered, and both the admiral and the captain which had manned the vessel were lost.
A. It triggered a distress signal, and an automated force had arrived and aligned itself to the protection of Colony.
B. An exploratory ship had discovered a cloning installation, with the capacity to create thousands and thousands of soldiers or workers. It was of alien design, but it was adapted for Man millennia ago.
C. To survive the plague, genetic augmentation research was spurned. It became common place for genetic and synthetic augmentations to be held on every colonist, but it came with a terrible, terrible cost. Their immune systems had started to fail, and a stop-gap had resulted in every man, woman, and child to wear bio-suits to separate them from the environment around them. Augmentations of the military variety had resulted in super soldiers becoming a fast reality.
D. The virus had... "activated" some individuals, resulting in some sort of latent "power" to become reality if they survived the infection.
E. An inactive Von Neumann Macro-Plague had been found in a Human research lab that was unearthed on the planet, it was stored and studied, while also replicated. The terrible weapon had no need for use, yet.
F. Something else? ((Be reasonable, please.)