“Level X” is episode nine of season one of Helix.
The Vectors (the infected) have disappeared from Level R and have turned up on Level X, a previously unknown level where lots of dangerous diseases are stored (sounds like a potentially dangerous place for them to be roaming around). Julia and Alan go down to try and retrieve the core sample of the virus.
Julia discovers more about her childhood. Some of this could have been guessed, by some of Hatake’s behaviour in the previous episode.