Whoop has something that shows warning that you can get ill, from illness before symptoms, so you should rest etc.
Key Indicators of Impending Sickness on WHOOP:
Red Recovery Score: A low recovery score (<33%) often indicates your body is under intense stress, such as fighting off a virus or bacterial infection.
Elevated Respiratory Rate: A significant deviation from your baseline respiratory rate is one of the strongest indicators of sickness, potentially highlighting illness, such as COVID-19, before you feel symptoms.