The Baal Shem Tov said that he learned all his remedies from the verse, "And He called to Moses…."(Heichal HaBracha, parashat Vayikra)
Besides being a mystic and a communal leader, the Baal Shem Tov was also a renowned healer, who used both natural remedies and segulot - supernatural methods - to effect cures. Rabbi Yitzchok Isaac of Komarno (Heichal HaBracha) explains that the letter alef at the end of the word "Vayikra" represents the level of keter and Ayin [Nothingness; Alef is the first letter of the Hebrew alphabet, and in this word, is written small in the Torah scroll, suggesting the idea of ego-nullification.] The Baal Shem Tov knew how to uplift the world to this supernal root and draw down healing energy.
chabad.org
The Noam Elimelech of Lizhensk. He was once visited by a poor farmer who had a 36-year old single daughter, but had no dowry to offer a young suitor in order to marry her off. Since it was a time of plague, Reb Elimelech decided to arrange a wedding for the woman, and took Lizhensk’s 30-year old bachelor water carrier and arranged for the Chuppah.
Some of the greatest early Chassidic luminaries attended: The Kozhnitzer Magid played the violin; the Chozeh of Lublin was the “badchan” (wedding jester). The dancing and singing were intense as everyone immersed themselves into the simcha. Reb Elimelech walked with his student, Reb Shmuel of Kariv, to see the celebration, and he noted that there was a great fire of holiness surrounding the entire assemblage. He remarked to his disciple: “Ribono Shel Olam, I declare that in the merit of this dancing that we have participated in, at least one hot coal awaiting us in Gehenna has been removed from all of us.”
This story indicates that it wasn’t the fasting and sackcloth that helped to alleviate the plague. It was rather the selfless rejoicing for another.
webyeshiva.org