Jorah Mormont is one of the most tragic and noble characters in the entirety of Game of Thrones. He has been loyal to Daenerys Targaryen longer than any of her other subjects, but that loyalty and love have come at great cost.
Though he started off as a spy for Varys, Jorah began to see what a great leader Daenerys was becoming. He pledged himself to her completely and developed a strong love for his Queen. Unfortunately, Daenerys discovered his spying and banished him from her service.
In order to get back in Daenerys’s good graces, Jorah kidnapped Tyrion Lannister in order to present him as a gift. While passing through the ruins of Old Valyria, the two were attacked by a group of Stone Men. These mutated humans carry the infectious disease known as Greyscale, an affliction that turns the host into a raving lunatic. Jorah contracted the disease while he was fending off the attackers.
After presenting Tyrion to Daenerys, Jorah confessed his love to his Queen and begged to be brought back to her side. Daenerys ordered him to find a cure for his disease and then return to her. Jorah made his way to the Citadel where the most educated maesters might be able to help him.
While there, he crossed paths with Samwell Tarly. Although Archmaester Ebrose did not want to operate on Jorah, Samwell took it upon himself to try and save Jorah’s life. Sam succeeded and Jorah is now on his way back to his Queen.
If you want to know more about Greyscale, check out this video: