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The Sisters of Plenitude were an order of Catkind nuns, devoted to helping the sick.
One order of the Sisters had a hospital on New Earth, outside of New New York. This hospital was created by the Sisters of Plenitude to deal with all human diseases. They had cures for nearly every known ailment and were able to heal their patients with remarkable efficiency. Many of the treatments shouldn't have been discovered for at least another thousand years, according to the Tenth Doctor.
These amazing skills were gained by using thousands and thousands of grown human clones. They were infected with every known disease, and later euthanised when too dangerously ill. This was done because other, more humane methods were too slow to deal with all the diseases. These disease carriers were released by Lady Cassandra and eventually cured by her and the Tenth Doctor. The Sisters were later arrested by the NNYPD for their actions. (TV: New Earth)
After their arrest, Novice Hame was forced to serve her penance as the Face of Boe's nurse, and was protected by him when the bliss virus killed the rest of the upper-world "in seven minutes", presumably including the imprisoned Sisterhood. (TV: Gridlock)