The Spira were percentage-obsessed aliens who duplicated beings and drained their life force, assimilating entire populations. They believed numbers yielded precision and precision yielded results.
The Spira could duplicate beings after paralysing them. It took two charges to duplicate a human, with the subject having to recover from the first charge before the second could take place. The first gave a basic body structure as a template, a half-formed Spira, with the second allowing the Spira to access characteristics and the subject's memory store. This killed the subject. The Spira could use its collective data however it wanted, providing statistics and dividing them among its brethren when they linked hands.
In 1665, the Spira came to make Earth their new home planet. They took over the village Red Lodge in Suffolk as an experiment area and gathered 100 human test subjects, including the Fifth Doctor and Nyssa. Not knowing they were human, Nyssa tricked the Spira into becoming the "human mean," genderless and statistically optimised. When the Spira made their Great Becoming, they each had and extra .01% heart from the Doctor, including all the accumulated biological differences from the Doctor and Nyssa, which lead to fatal anomalies in the Spira's new bodies and they died. (AUDIO: The Demons of Red Lodge)