The interphasic organisms were parasitic lifeforms native to Thanatos VII that fed on cellular peptides. They existed in an interphasic state that rendered them invisible and intangible, unless illuminated using an interphasic scanner. They could not be physically removed and were unaffected by electromagnetic radiation, subspace fields, or thermal protons.
In 2370, the crew USS Enterprise became infested with these organisms, which arrived on board inside a new, recently installed warp plasma conduit. The conduit had been manufactured on Thanatos VII using a new interphasic fusion process, which drew the organisms to it. The organisms then lay dormant within the conduit until the Enterprise crew activated the warp core.
The infestation would have eventually proved fatal to the Enterprise crew. Fortunately, Lieutenant Commander Data's positronic subprocessor detected a high-frequency interphasic signature from the organisms. Data's subconscious mind perceived the threat they posed, which manifested as disturbing symbolic imagery within Data's dreams. By directly accessing these dreams, Data determined that the organisms could be harmed by a high-frequency interphasic pulse. Commander Geordi La Forge modified Data's subprocessor to emit the pulse, which successfully eliminated the creatures. (TNG: "Phantasms")