Here is a playable extinct alien species introduced in Star Trek: Enterprise, in the episode called “Extinction“. The Loque’eque had died out on their home planet in the Delphic Expanse, but they had engineered a mutagenic virus designed to transform other sentient alien species into the Loque’que. Once transformed, they suffer hallucinations and a biological imperative to return to their capital city, Urquat. Alien cultures encountering the virus were actively killing off the infected, fearing that the rapid spread of the airborne virus could erase their own species and replace them with the feral Loque’eque. When Enterprise visited the planet, some crew were likewise infected, but it turned out that Vulcans were immune due to their K-cells. An antivirus was created by Dr. Phlox to combat the virus. Phlox asked Jonathan Archer if the last virus sample should be destroyed, but Archer decided that it shouldn’t. It was the final effort of a species to preserve itself, and he didn’t want to be the one to cause their extinction. He asked Phlox to save the sample, to keep it in stasis.
This leaves the possibility that the Loque’eque virus is still around in some form. Perhaps the Federation found a way to restore the Loque’eque without activating the contagious aspect. Or, if you are using the E² materials this month, the time-shifted Enterprise may still have a virulent sample aboard. Without modern shields, it could easily be stolen by another alien ship using transporter technology. What would happen if the virus was released into the air?
The PDF gives rules for the Loque’eque, including a special Talent for infected aliens turned into the Loque’eque.