Make it standard for sick passengers to sit in the last row of the plane to avoid infecting more passengers.
Often people travel on planes sick. It is understandable that if someone has booked airplane tickets, hotels, etc they do not want to change/cancel their plans. However, even with the protection measures in the era of a pandemic, transmission can still take place on a plane. People do not always wear reinforced masks and during meals, everyone removes their masks at the same time.
Even if a sick person does not have Covid (which may also be the case even if they are vaccinated/tested etc), there are thousands more viruses someone can catch. It is not fair for other passengers to get sick, and potentially have their own vacation disturbed.
1) The person who is unwell informs the personnel at the check-in counter or the gate before boarding. The personnel makes arrangements for the sick passenger to be seated at the back row and they try to leave empty as many seats as possible around them. If the plane is full they can transfer some passengers from the last rows to business class ;-)
It would be smoother if the person could already indicate they are sick during online check-in so that the seats are arranged beforehand. However, it would take time to implement this feature in the online check-in process.
2) Another passenger informs the flight attendant that someone sitting close to them are sick (sneezing, coughing, shivering . . .). The flight attendant asks politely that passenger to go to the back of the plane. Alternatively, they do not move the sick passenger but the people who sit next to them and want to be transfered.
In any case, the sick pessanger needs to be given a high protection mask.