When the Doctor and Rose Tyler arrived on New Earth, the Doctor had a message to meet the Face of Boe in Ward 26 of the hospital. Before meeting the face, he passed by the Duke of Manhattan, who was turing to stone. When the Doctor looked for Rose, he saw that the Duke was cured of his disease. A Sister of Plenitude explained that they healed him.
The Doctor finally discovered what the Sisters were up to: deep under the hospital was a facillity where the Sisters of Plenitude were creating humans and infecting them with every disease in the galaxy, experimenting on them to find a cure.
The Lady Cassandra(who has taken control of Rose's body) knew the Sisters were up to something and when she found the humans, she threatened to tell their secret. The Nuns were about to claw her to death when she released the humans and the nuns ran.
The Doctor combined all known medicens and sprayed the humans, curing them. The Sisters of Plenitude were arrested.