Memory virus
The memory virus was a virus that passed from person to person through physical contact. A memory virus inhabited the mind of Dimitri Valtane until 2293, when it moved to Tuvok shortly before Valtane's apparent death.

To avoid detection, the virus disguised itself as a memory engram – normally a memory so traumatic that the host suppressed it into the subconscious. In Tuvok's case, the memory was of a girl falling from a cliff. 

The virus was discovered and destroyed in 2373 by The Doctor aboard the , using thoron radiation. ()

Microorganisms