A better early indicator of covid is an oximeter - cheep device available at walgreens that measures the percent of oxygen in your blood - clips onto your finger. They're finding that oxygen starts dropping as your lungs are affected even before fever or shortness of breath.

Had a scare myself last week when my temp spiked from normal to 104 over 20 minutes combined with shivering and chills. Talked to my doc and he sent me to the ER. Covid came back negative (new 2-hr tests) but a CT showed I had a urinary track infection that backed up into my kidney - not cool but a couple weeks of antibiotics and I'll be good.