Season 3, Episode 5:
Danny Quinn breaks into the ARC and is drafted in to help the team when a fast-growing flesh-eating fungus from the future is transferred to the present and infects a businessman, Sir Richard Bentley. Danny arrives with flamethrowers to try and burn Bentley as he is already infected not realising fire doesn't work. The team manage to bring the creature back to the ARC to destroy it, where Jenny is trapped in the freezing cold main operations area with it and forced to fight for her life. After her recent discovery of Cutter's picture of Claudia Brown and her near-death experience, Jenny decides that she's had enough: she resigns from her job at the ARC, leaving Danny in charge of the team.
To learn more about the creatures of "Primeval," watch the installment of "More" that's associated with this episode here.