Gastropomorphic virus

The gastropomorphic virus was a contagious virus which turned its host's body into that of a large slug-like creature. It was discovered by the Federation around 2398 aboard the USS Paramaribo. 

The virus could only survive in Vulcanoid hosts, such as Vulcans and Romulans; it did not affect other species, such as Humans, Caitians, or Kasheeta. Because the transformation process turned the host into a non-Vulcanoid creature, it would eventually die off when it did not have any other hosts to spread to.

Jeremy Amundson was infected with the virus around 2398, but was later restored to his normal form by using the transporter. C. A. Kelper suspected that the virus was genetically engineered by an enemy of the Romulan Star Empire.