Seamus O'Dell was a Human Augment who lived on planet Tagra IV in the late-24th century. He was a part of the group led by Rhys Darcen that had broken away from Section 31 after the agency had considered, but then declined to use biological agents in the Dominion War.
When Darcen's group took samples of the virus to use on their own, O'Dell volunteered himself as a test subject while using the Federation listening post of Epsilon Trianguli as a proving ground. All the others assigned to the listening post died from the virus, but O'Dell survived. His genetic enhancements allowed him to stay conscious long after life support had failed, although he was in the early stages of hypoxia when a team from the USS Lambda Paz arrived at the scene. (Star Trek: Lambda Paz: "Midnight Ride ", Religion To Do Good")
O'Dell would later escaped the Lambda Paz with Chief medical officer Aurellan Markalis, who was assigned to infiltrate the augments, returning to Tagra IV to reunite with the rest of Darcen's group. Markalis was then instructed to take samples of O'Dell's antibodies in order to modify the virus so that augments were immune.
The virus would eventually run its course, resulting in O'Dell's death. Though he was a long-time friend of Darcen's, Darcen did appear indifferent to O'Dell's inevitable demise. Darcen even insisted that O'Dell not be administered palliative treatment, believing that would needlessly prolong his life. ("Religion To Do Good")