Quinn played a crucial role in the uncovering of an attempt to take over Starfleet Command by neural parasites in 2364. Quinn ordered the Inspector General officer Dexter Remmick to investigate whether or not the USS Enterprise-D had been infiltrated also. (TNG: "Coming of Age")
Nothing was discovered at the time, but Quinn himself would later become a victim of alien infestation. His mind was overtaken by the unknown alien intelligence in an attempt to infiltrate Starfleet Headquarters in 2364. Later that year, Captain Jean-Luc Picard and Commander William T. Riker of the Enterprise-D destroyed the alien mother creature, which lived in the body of Dexter Remmick, thereby also killing all other alien parasites on Earth. Quinn himself was expected to make a full recovery following the ordeal. (TNG: "Conspiracy")
In the year Admiral Quinn was infected, he was attached to Starfleet Operational Support Services and made among others the executive decision to postpone the planned upgrade of the warp sensor grids of the USS Gremlin due to budgetary considerations. (TNG: "Conspiracy")
Quinn was never seen or mentioned again after the first season of The Next Generation, with the exception of archive footage used in TNG: "Shades of Gray". Quinn's recovery is stated in a log entry made by Captain Picard.
Quinn returned, one rank higher, as a full rear admiral in issues of DC Comics' TNG series story arc entitled War and Madness.