This is dysentery.
It all began with an epidemic of sickness on the Ridge, which Claire diagnoses as amoebic dysentery.
“For Claire, the ether is not addictive in itself, but the feeling it gives you is. The idea of putting yourself to sleep and not having to deal with any of these feelings she's having inside, that is something Claire will become addicted to.”
To be more specific, it is amoebic dysentery, which Claire (Caitriona Balfe) confirms with her makeshift microscope. The disease can be spread through contaminated food or water, and Jamie (Sam Heughan) eventually learns that it was a dead elk lying in the water that the fisherfolk use that caused the disease.
After attending to Claire 24 hours a day for almost a week, Malva and housekeeper Mrs Bug (Sarah Collier) decided to cut her hair off, thinking it would help reduce Claire's temperature. Having to rock a much shorter hairdo for her character, Balfe, revealed she “wasn't a big fan,” of the shorter look.
Jamie denies having slept with her, but the damage to his and to Claire's reputation is done—and the bond between the Frasers and Malva is severed, until just a few weeks later, when Claire finds Malva murdered in the garden.
Allan began molesting his half-sister Malva from the time she was a young child and eventually, gets her pregnant.
Later, after Claire has returned to the Ridge, she is confessor to Allan Christie, who explains that he was the one that fathered Malva's child, and then murdered her.
It turns out that after recreating ether, an early form of anaesthetic, in the era she now lives in with Jamie (Sam Heughan), she's begun to self medicate to help her sleep.
Claire becomes deathly ill with dysentery (this is clearly not her year). While she's passed out, Mrs. Bug and Malva Christie (more on her later) shave her head to lower her fever.
Jamie finally saw her as she tried to fight against using the ether. That led to Claire having no choice but to open up about her struggles, and it brought us a moment that pulled Claire and Jamie closer. Jamie wasn't angry about Claire using the ether. He listened to her, and understood the feelings she held.
The good news was that Claire and her baby survived thanks to the poison being "bitter cascara," which appeared to be nonlethal.
“I curse you. I curse you with knowledge, Jack Randall. I give you the hour of your death. Jonathan Wolverton Randall, Born Sept 3rd, 1705, dies…” she whispers in his ear, and suddenly he knows that she's telling him the truth.
Director Brendan Maher told Vulture that while both Menzies and Heughan had stunt doubles for the physically demanding sequence, neither used them much. “We had people for safety, just in case, but the nature of the work is that you want the actors to do as much as they possibly can,” Maher said.
Claire fortunately recovers from the ordeal and realises both she and Tom did not have dysentery.
The nurse, played by Caitriona Balfe, creates the painkiller ether and self-medicates with it as she tries to get over her traumatic experiences at the end of season five. Ether is a drug that was used in the 20th century as an anaesthetic during operations but also found fame as a recreational drug.
About half way through the book, the protagonist, Claire, is violently beaten by her lover/husband, Jamie, as punishment for endangering him and his clan.
We've seen Claire throughout the season have numerous hallucinations of Lionel Brown. In this episode, her hallucinations of Lionel frequently allude to witchcraft, selfish tendencies, or exploit the anger and betrayal she felt over Malva's false accusation of Jamie.
Claire's initial intention is to use the ether to help her safely operate on patients. But as she experiments and begins testing it on herself, she finds "a comfort to the escapism that it gives her, that she begins to rely upon," Balfe says.
Claire eventually realizes that Malva is lying because she knows her husband well enough. Tom seems to question Malva's story.
Tom and Claire unexpectedly meet each other with the preacher questioning whether Jamie Fraser (Sam Heughan) was aware of his feelings. In the book, Claire confirms Jamie is well aware of Tom's unrequited feelings towards her and understands them as someone who is in love with her.
Mona was sentenced to hang, leaving both Allan and Malva in the care of their aunt.
However, Ian really did sleep with Malva—but he was not the only man Malva lay with.
Allan told Malva to cover up the child's true paternity by pointing the finger at Jamie, believing he would take pity on the unmarried young girl and at least pay for the infant. However, it all turned sour when Malva wished to reveal all to Claire with Allan killing her before the truth could be uttered from her lips.
We find out who did it in A Breath of Snow and Ashes, but it takes some time. Claire is arrested for murder, but Tom learns the truth in the end. It was Allan who did it. It turns out Allan sexually abused and raped Malva, and he is the father of the child.