Memory virus
From Memory Alpha, the free Star Trek reference.
The memory virus is a (possibly alien) virus that passes from person to person through physical contact. A memory virus inhabited the mind of Dmitri Valtane until 2293, when it moved to Tuvok shortly before Valtane's apparent death.
To avoid detection, the virus disguises itself as a memory engram -- normally a memory so traumatic that the host suppresses it into the subconscious. In the case of the one memory virus known to Starfleet, the memory was of a girl falling from a cliff. It is not known how many hosts the virus inhabited before it was destroyed, but it may have been in existence for thousands of years.
The virus was discovered and destroyed in 2373 by the Doctor aboard the USS Voyager, using thoron radiation. (VOY: "Flashback")
