A quarantine is used to separate and restrict the movement of persons; it's a state of enforced isolation.
In the franchise
In Alien, Ripley warns the crew members to citing quarantine regulations, when Kane is attacked by a facehugger and Dallas and Lambert want to take him aboard. Out on the hostile environment of LV-426, Ripley refuses to let them aboard, but they eventually do, because of Ash violation of protocol.
In Prometheus, after discovering that Shaw is pregnant, even knowing she is sterile, the crew adopts quarantine regulations. Holloway, love interest of Shaw, was infected by the black goo. The pregnancy of Shaw indicated that the black goo was some kind of 'evolutionary fluid' that made possible the creation of a form of life that was growing inside Shaw's body. In order to avoid some possible infestations, Shaw is set in quarantine measures.