Memory Virus, Flashback

A microscopic picture of the memory virus

Memory virus on Janeway's engram

Overlaid in red on Janeway's memory engram in 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 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.