The meme virus was a type of telepathic disease, a thought pattern that could be placed as a stray thought in the mind of a telepath, being transmitted during various types of intermittent or sustained telepathic contact between two minds. It was difficult to detect, as it only existed as a minor brain wave pattern within the overall body of a victim's mental and nervous brain-wave makeup. Because the thought pattern could lay dormant in a mind, it also was difficult to tell who was infected until it was too late.
As the infectious thought inhabited a host's mind, it would spread virally, causing other thought patterns to interact with it, rewriting and mimicking pre-existing thought patterns such as instinct and learned behavior. As the other thought patterns changed over to the new form, extreme disruptions in the individual's well-being would occur.
The Dominion had engineered a disease of this sort to disrupt the Federation and other Alpha Quadrant governments as a pre-emptive move to weaken galactic solidarity against them during the buildup to the Dominion War, in 2373.
Different telepathic races and cultures were affected differently, owing to differing brain structures. For example, Vulcans and telepathic Romulans would lose conscious control of their emotional state, maddening them to a state of extreme hostility. Betazoid and Human empaths would experience a reversal of their empathic sense, with a shift in polarity causing them to no longer recognize emotional communication correctly. When half-Human, half-Betazoid Deanna Troi was infected, her empathic sense shifted, causing her to think that those who exhibited caring, friendly emotions were actually malicious. She found herself gravitating towards hostile and negative emotional contact, which she misinterpreted as genuine familiarity and acceptance.
Federation medical researchers came upon the cause for the as-yet unexplained aberrant behaviors in telepaths quite by accident. A Ferengi cadet, Nog, was assigned as an orderly in Deep Space 9's infirmary and found that he had a seemingly familiar song in his head that he could not remember the origin of. The continued attempts at remembrance caused him to become fixated upon it, and he whistled it to himself while he worked. When Julian Bashir became distracted by the music's repeated nature as Nog walked by, he asked him about it, and had a flash of insight as to the transmission of the meme virus—as a transmitted thought that would not go away, like a song stuck in your head.
Cadet Edam Astrun was found to be immune, as he had absorbed an immunity during mental contact with a Changeling infiltrator. Astrun, who was working with Deanna Troi, was able to reach the Talosians, who had contacted him with a telepathic distress call. With the help of their powerful mind broadcasts, the entire Federation was contacted with a sort of mental inoculation, foiling the Dominion's plot.
The Xoxian civilization was devastated by this attacking mind-plague after a Federation delegate, Uxumel, returned to his world to attend to his ailing daughter, carrying the meme to his world. The people of Xox were completely dependent on telepathy for survival, it was their main sense. All native Xoxians were born without eyes, ears, or speech, instead relying on telepathy to communicate language and perceive the world around them. When the meme attack effectively blinded, deafened and muted a majority of the Xoxians, the shock killed many, and many more died accidentally from being totally disoriented from the world around them. A great many also chose suicide over the hopeless torment of being suddenly and completely insensate.(TNG - Telepathy War comic: "Heart of Darkness"; ST - Telepathy War comic: "Reality's End")