How do I get rid of constant thinking about diseases and the fear of death because of them?

What you have mentioned about thinking of illnesses and fearing death may be due to a distance from Allah. Therefore, be connected to Him, and know Him in prosperity so He may know you in adversity. Turn to Him with sincerity and devotion through supplication, and frequently recite: "There is no god but You; glory be to You! Indeed, I was of the wrongdoers." Allocate time for reciting the Qur'an and pondering its meanings, for Allah Almighty says: "And We send down from the Qur’an that which is a healing and a mercy to those who believe." (17:82) And treat your anxiety and sorrow with the supplication narrated from the Prophet, peace and blessings be upon him: "O Allah, I am Your slave, the son of Your slave..." (the rest of the hadith). Know that what a Muslim should truly fear is what comes after death, and this necessitates adhering to the limits set by Allah.