The term phage (ファージ, fāji?) occurs only in the anime. It describes any monster of the week formed by the use of Galaxia's golden armlets which are infused with the power of Chaos and are used by a Sailor Animamate. When the shine of a person's starseed dies, his or her body becomes a sort of parody Senshi (gender being retained). They change shape based on their dominant personality trait or hobby, and take on exaggerated mannerisms and names such as "Sailor Guts", "Sailor Teacher", or "Sailor Antique".
In the anime, after an animamate has extracted the starseed from a normal person they would often abandons each of them upon discovering that the victim only has a "normal" starseed before their brightness fades away. A person's starseed will then turn black as it corrupts and transforms the person into a phage. After fighting the Phage, Eternal Sailor Moon has the power to restore them to their normal forms and to return their normal starseed back into their human bodies using the Starlight Honeymoon Therapy Kiss attack and the Silver Moon Crystal Power Kiss attack. In the anime Princess Kakyuu is implied to have this power as well.
It should be noted that those who possess a true star seed or a Sailor Crystal do not turn into a phage because they hold the true star seeds that eternally shine. An example was during Episode 193, when Sailor Lead Crow forced Usagi to surrender her star seed. After extracting it from her, surprisingly she does not turn into a phage as she possessed what Lead Crow considers to be a 'true' star seed or a Sailor Crystal that is strong enough to shine longer. However, when starseeds are taken from a Sailor Senshi or a normal person, their body fades away killing them instantly. The anime, however, never explained the true origins of phages but it is assumed that they are the products of Chaos' power which was infused onto Sailor Galaxia's golden bracelets and were also given to the Sailor Animamates in exchange for their star seeds. This power within the bracelet may have an influence towards the formation of phages from victims who's starseeds are extracted but are not taken by the Animamates.
While phage do not appear in the manga or musicals, they are referenced in one of the songs in the musical.
Read more about this topic: Shadow Galactica