“Single Strand” is episode four of season one of Helix.
At the end of the previous episode, “274”, Dr. Julia Walker realised that not only had she been infected, but also that the test they had devised to spot infection didn’t work. Major Balleseros had destroyed the satellite dish, cutting the base off from outside communications for some reason and also apparently damaging internal communications. With the infected sealed off, and communications down, Julia had no means of communicating the fact.
What with the lift music at the title screen going wrong (deliberately) and the infected scrambling around corridors this is getting very survival horror.