F-four

 was a human colony. At the time the Fifth Doctor, Nyssa and Tegan visited, the colonists had been there ten years.

The colonists were unable to have children. With the Doctor and Nyssa's help, they discovered that, prior to their arrival, someone had left a virus on the planet. The colonists would be carriers of the virus, which was a disease deadly to humans. The government on Earth, realising this had happened, sent seeds to the colony that contained a contraceptive, preventing the colonists from having children.

Because the disease was so deadly, the Doctor couldn't remove the colonists from F-four. When the travellers left, they promised to tell others the colonists' story. (PROSE: No Exit)