Star Trek: 10 Classified Starfleet Secrets
5. The Morphogenic Virus
One of the most abhorrent acts ever committed by Section 31 was their creation and deployment of the morphogenic virus.
Section 31, realising that the Federation's hopes of winning the Dominion War were dwindling, intentionally infected Odo with the virus during a medical examination. The top-secret morphogenic virus was specially designed to kill Changelings, the leaders of the Dominion. They hoped that Odo, when linking with other Changelings, would pass it on to the whole population.
Odo was never meant to develop symptoms, but when he began to, Doctor Bashir did some digging and exposed the Section 31 plan. Using some fancy tech, Bashir was able to enter the mind of the Section 31 agent, Luther Sloan, to extract the cure, saving the Changelings, including Odo, and improving the relationship between the Dominion and the Federation.
Many in the Federation opposed giving away the cure to save their enemies, but this act of mercy was instrumental in ending the Dominion War.