Memory virus

A microscopic image of the memory virus

Virus engram (red) overlaid on Janeway's memory engram (white)

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 USS Voyager, using thoron radiation. (VOY: "Flashback")

As of 2401, a sample of the memory virus was stored on Daystrom Station. (PIC: "The Bounty")

As evidenced by the first draft script of "Flashback", what ultimately became the memory virus was originally to have been commonly referred to as an alien parasite instead, with no mention of it as a virus.