The memory virus was a (possibly alien) virus that passed 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 disguised itself as a memory engram – normally a memory so traumatic that the host suppressed 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.