The Twenty-fifth Amendment states (in part):
Section 1. In case of the removal of the President from office, or of his death or resignation, the Vice President shall become President.
Section 4. Whenever the Vice President and a majority of either the principal officers of the executive departments or of such other body as Congress may by law provide, transmit to the President pro tempore of the Senate and the Speaker of the House of Representatives their written declaration that the President is unable to discharge the powers and duties of his office, the Vice President shall immediately assume the powers and duties of the office as Acting President.
The White House reported in October 2020 that former President Trump had tested positive for COVID-19. If a future President developed severe symptoms of this (or any other) disease, and was unable to continue working, what would happen then?
Here are some resources that discuss the 25th Amendment to the U.S. Constitution, which explains the procedure for replacing a president who is unable to carry out his or her responsibilities due to removal from office, resignation, illness or incapacity, or death.