And the body can focus on fighting the disease and not digestion, which is work, too. I am currently sick and I just don't want to eat (much), so I won't. That's why we have fat reserves.
Anecdotally, several people have told that they lost a few pounds of fat while suffering from mild COVID-19 symptoms despite still eating during the course of the disease. And this was real fat loss, not just dehydration. Fighting the infection seems to burn a lot of calories.