Answer by taking a quote from the following article:

Abu Bakr as-Siddiq 'Abdallah bin Abi Quhafah (Arabic: 'bw bkr lSdyq `bd llh bn 'by qHf@; c. 573 CE - 22 August 634 CE), popularly known as Abu Bakr ('bw bkr), was a senior companion (Sahabi) and--through his daughter Aisha--the father-in-law of the Islamic prophet Muhammad. Abu Bakr became the first openly declared Muslim outside Muhammad's family. Abu Bakr served as a trusted advisor to Muhammad.

Abu Bakr attended the event of Ghadir Khumm, which took place a few months before Muhammad passed away. According to both Shia and Sunni sources, he was among the many who pledged allegiance to Ali at the event.  In Medina, after the Farewell Pilgrimage and the event of Ghadir Khumm, Muhammad ordered an army under the command of Usama bin Zayd. He commanded all the companions, except for his family, to go with Usama to Syria to avenge the Muslims' defeat at the Battle of Mu'tah. Muhammad gave Usama the banner of Islam on the 18th day of the Islamic month of Safar in the year 11 A.H. Abu Bakr and Umar were among those that Muhammad commanded to join Usama's army.  However, Abu Bakr and Umar resisted going under the command of Usama because they thought that he, who was 18 or 20 at the time, was too young to lead an army, despite Muhammad's teachings that age and standing in society did not necessarily correspond to being a good general.  In response to these worries, the Prophet said: "O Arabs! You are miserable because I have appointed Usama as your general, and you are raising questions if he is qualified to lead you in war. I know you are the same people who had raised the same question about his father. By God, Usama is qualified to be your general just as his father was qualified to be a general. Now obey his orders and go." Whenever Muhammad felt any relief from his fatal sickness, he would inquire as to whether Usama's army had left for Syria yet, and would continue urging his companions to leave for Syria. Muhammad even said, "Usama's army must leave at once. May Allah curse those men who do not go with him." However, while a few companions were ready to join Usama's army, many other companions, including Abu Bakr and Umar, disobeyed Muhammad's orders. It is also noted that this was the only battle expedition where Muhammad urged his companions to go the battle no matter what; for other battles, if someone was unable to go to the fight, Muhammad would let them stay at home.

What led to his death?
Whenever Muhammad felt any relief from his fatal sickness,