According to traditional accounts, Tatars poisoned Genghis Khan's father, Yesügei. The story appears most famously in The Secret History of the Mongols, the key surviving narrative source for Temüjin's early life.
Yesügei was a leading chief of the Khamag Mongol confederation and had previously fought the Tatars, including killing a prominent man named Temüjin Üge. While returning from arranging his son Temüjin's betrothal to Börte, Yesügei encountered a Tatar wedding feast. The account says the Tatars recognized him, concealed their hostility and mixed poison into his food.
Yesügei reached home but became seriously ill and died around 1171, when Temüjin was about nine years old. His death left Hö'elün to raise their children after Yesügei's followers abandoned the family, a hardship that shaped Temüjin's rise.
This answer describes a traditional account, not a medically verified diagnosis. The source gives poisoning as the cause and attributes it to Tatars, but the precise substance is unknown. “Tartars” is an older English spelling of the same group name in this context; it should not be confused with every later people called Tatars.