Chandler needs Rachel to talk to Niels in order to get his help to nebulize the cure so that they can spread it to the masses. The two them figure out that they need a particular spore found in fresh water muscles to be able to give the cure the stability it needs but Niels tells her he doesn’t remember the exact sequence to make it happen. She knows he’s lying. Later, when she realizes he’s attracted to her, she uses it. Rachel makes him believe he has a chance with her and he shares the sequence. Rachel later realizes that the way to transmit the cure is to make it contagious, the same way the virus is contagious as it lives in Niels. She deduces that she can use that strain that resides in Niels' lungs to make that happen but the virus needs to leave his lungs in order for that to happen and in doing so, it will kill him. So Rachel gives him an IV that, unbeknownst to him, causes the virus to spread throughout the rest of his body, killing him.
Watch The Last Ship Season 2 Episode 9 Online here at TV Fanatic.