Virus

A virus works by inserting itself in a healthy cell and replicating it's own DNA and taking over the vector cell. The only way to kill a virus is to kill the cell it inhibits.

A virus inhabits a cell and keeps the exterior, but replaces the interior. A patient who has a virus is repeating a situation, but utilizing a new behavior.

When Meredith and Derek inject the tumors in the clinical trial with a virus, they are taking an issue from their past, and approching it with a new behavior. The virus does not work until they operate on Beth. They are navigating their issues and trying to take the appropriate approach to destroy thier issues (tumor) however, they are not able to appropriatly adapt their behavior until the end.