Colds usually stay "above the neck." Symptoms develop slowly over a few days. The main symptoms are a runny or stuffy nose, sneezing, and a scratchy sore throat. The intensity is more mild. You might feel "under the weather," but you can usually go about your day. A fever is very rare in adults.

The flu is notorious for hitting you all at once. You might feel fine in the morning and be bedridden by the afternoon. The main symptoms are high fever of 100º to 102º, severe body aches, and "bone-deep" exhaustion. The intensity is high. It often feels like you’ve been hit by a truck. A cough (usually dry) can be quite painful or persistent.

COVID-19 symptoms can look exactly like a cold or flu, but they often last longer and can affect other systems in the body. The main symptoms are fever, dry cough, and shortness of breath. Unique markers are a sudden loss of taste or smell (though less common with newer variants) and gastrointestinal issues like nausea or diarrhea. The onset of symptoms can appear anywhere from 2 to 14 days after exposure.