Dead Space Wiki
The quarantine must have tripped when the filtration system restarted, everybody relax.

Zach Hammond[1]

A Hazardous State Quarantine was a defense mechanism built into the USG Ishimura, USM Valor and Aegis VII Colony and was often the first and last lines of defense in the attempt to contain a disease or fugitive. A similar system existed on the Sprawl and on the New Horizons Colony.

Ship-wide and Colony sensors and security systems constantly sweeped the areas where contamination by a foreign substance or lifeform was both likely and extremely dangerous to the people in the area. On detection of a foreign contaminant or hazardous biological anomaly, security systems would seal all doors and place Blast Shutters to contain everything in the room. Quarantines would remain active until termination or removal of the anomaly occurred.

Much to the chagrin of Isaac Clarke and his companions, quarantines are very commonly encountered during their time on the Ishimura adding challenge to already dire situations. While these safety measures are exceedingly effective at containing human threats, they did little to hinder the movements of the Necromorphs who used the system gap to their advantage by dropping from air vents without warning. This ambush tactic combined with the darkness that befell a quarantine zone made it nearly impossible to see such enemies like Enhanced Slashers and Lurkers until it was too late and often trapping Isaac with a horde of Exploders.