In the year 2284, the sixteen Prime Worlds were colonized by the Earth Empire. One of those worlds, and the 11th to be settled, was the planet Seton. Classified as a 'Paradise' world, its environment, atmosphere and weather were almost identical to those found on Earth.
Historical records from that era indicate that despite Seton's lush environment, life was particularly tough for the first human settlers. This was due in part to the settler's general inexperience in creating colonies, but the lack of indigenous animal life larger than a small insect also played a role.
In order to address the colonist's dietary needs, DNA patterns taken from Earth animals were transported to Seton. Soon, not only were various forms of livestock present on the planet's surface, but also Earth corn, wheat and soybeans. A random sampling of cargo manifests indicate that record amounts of grain was exported from Seton during this time.
However, after 15 years of problem-free life, the colonists on Seton began to die from a variety of maladies. Most medical records from this era remain sealed, but a scan of pharmaceutical-related cargo manifests demonstrate a substantial increase in medicines designed to combat such illnesses as pneumonia, Crohn's disease, and Burkitt lymphoma, among others.
Dubbed by the colonists as "Seton's Curse", the wave of illnesses quickly crippled life on Seton. With the death toll rising, Earth halted all imports and exports from Seton and quarantined the world.
Two years later, with fatality rates hovering around 50%, scientists discovered that a particular Seton-based mold, when combined with certain Earth fungi and foodstuffs, mutated into an 'enabler' that suppressed or enhanced random immune functions. Both a treatment and vaccine were soon ready and life on Seton return to normal.
The most significant impact from the episode on Seton was the creation of strict protocols that governed the testing of a planet's potential health-hazards and how and when off-world plants and animals should be introduced to a non-native ecosystem.