What happens if a president is unable to serve because of illness?

The Vice President takes over until the President is well enough to
serve again. If the President does not recover the VP is sworn in as President.

This is how Lyndon Johnson became President. John Kennedy was shot and did not recover so Johnson was sworn in.

This order of replacement or command is Presidential Succession.

It is meant that like tapestry scientific theories often need to pull out since they are not relevant and often times the whole chain of theories may be pulled out.