A cryogen tube is a life sustaining device used in Life Science stations.
Such tubes are capable of monitoring and suspending a patient's life functions. Such tubes are also used as a form of isolation, preventing pathogens from infecting life sciences practioners or med techs as they treat them.
Presumably, the cryogen tube works through some form of freezing, given that ice crystals form on patients inside the tube.
Those infected by the virus brought back from the Otarsis Quadrant asteroid are placed in these chambers (Lost Planet of the Gods, Part I).