A Quarantine Module was a module that was used by Starfleet at transit points for the treatment of highly contagious diseases that crews encountered in space.
Quarantine modules were often self contained units, capable of existing outside the structure of a starbase in open space. Normally, in such cases they were connected to the starbase via an umbilical line. However if desired the unit could be modified by placing life support units in the quarantine module, allowing for the module to be disconnected from the main structure. This permitted the physical isolation of people carrying contagious diseases.
Shortly after the United Federation of Planets assumed control of Deep Space 9, station personnel began constructing a quarantine module - using an abandoned Cardassian ore transport as the backbone for the module. It was discovered that the Cardassians were intending to construct a substation and manuever it into position at the Gamma Quadrant end of the Bajoran wormhole to claim that end of the wormhole. To secure the Bajoran claim to both ends of the wormhole, the quarantine module was converted into a space station so that the Bajorans could monitor traffic and establish a Bajoran claim to the space around that end of the wormhole. (DS9 novel: Bloodletter)