If you are sick, you need to provide the materials required by the airline. Take Sichuan Airlines as an example. Refund requires hospital certificate (original), sick leave certificate (original), invoice and prescription sign issued at or above the county level. Different airlines need different materials, just provide materials according to relevant policies.
The Civil Aviation Administration announced that people who are not suitable for flying include pregnant women (more than 32 weeks pregnant but not more than 35 weeks pregnant), newborn babies, seriously ill patients, patients with infectious diseases, mental patients or passengers whose health may endanger themselves or affect the safety of other passengers. If these conditions are met and relevant cases are issued, the airline will determine that it is not suitable for flying. Diagnostic proof of unfitness for flight. Doctors in general hospitals will not issue such certificates, and diagnostic certificates that meet the above conditions are not suitable for flying.
So don't panic when you are sick. Refund policy can call the airline customer service. There is no diagnosis that it is not suitable for flying. As long as you are really sick and can't fly, you can refund your ticket smoothly.