What do airlines do to prepare if an individuals travel plans are compromised by Covid?

Unfortunately I don't think they'll do anything. Normally if you're too sick to travel that's a matter for travel insurance. You could try contacting them before the flight and ask politely whether you can reschedule. You might be able to cancel anyway if you only recently booked. I accidentally booked a flight for the Xth of the wrong month and was able to change it on the day at the airport for a small admin fee. Another time I booked a flight I didn't need just for proof of onward travel and I was able to cancel for a full refund on the same day. Another time I just changed my mind and got back virtually nothing, just the taxes. All the airlines have different rules and there's some room for discretion.