You were key in stopping a plague from overtaking Paragon City.
How to Get
Earn the Plague Stopper accomplishment badge by completing the Destroy Vahzilok Containers mission from Olivia Chung, Dr. Ann-Marie Engles, Juliana Nehring, or Pavel Garnier.
In Ouroboros, as a Flashback mission, you must complete the "Vahzilok Wasting Disease" mission in the Guardian (Level 16-19) section.
The Plague Stopper badge is awarded to characters of the hero and vigilante alignments. The same badge on villains and rogues is called Deadly Virus.