As winter approaches, many people wonder how to tell if they have a common cold, influenza (the flu) or COVID-19. These are all caused by respiratory viruses, and they share some of the same symptoms, but they can lead to very different outcomes. Several other illnesses, such as respiratory syncytial virus (RSV) and mononucleosis (caused by Epstein-Barr virus), can also cause similar symptoms.

The common cold can be caused by a variety of viruses, including rhinoviruses, coronaviruses and adenoviruses. Typical symptoms include sore throat, nasal congestion (a stuffy or runny nose), sneezing, coughing and headache; fever is uncommon.

Influenza can cause these symptoms too, but more often it presents with fever or chills, fatigue, headache and muscle or joint aches. Some people may have gastrointestinal symptoms. Flu symptoms tend to come on more suddenly and are typically more severe than cold symptoms.

COVID-19, caused by the SARS-CoV-2 coronavirus, can lead to a wide range of symptoms. Early virus variants typically caused fever, a dry cough and shortness of breath; one distinctive feature was loss of the sense of taste or smell. However, the newer omicron variants cause symptoms more like those of a common cold, including fatigue, sore throat, congestion and headache. People who are vaccinated are more likely to have mild symptoms or be asymptomatic.

Testing is the only sure way to tell which virus you have. Traditionally, people with mild to moderate cold or flu symptoms usually didn’t get tested, but COVID testing is recommended if you have symptoms. A PCR test administered by a medical provider offers the most accurate results but self-administered rapid antigen tests are more convenient and less expensive.

Common colds usually resolve in about a week without treatment. Most people with influenza or COVID recover within a few weeks, but these viruses can lead to severe complications. Older people are more likely to have severe flu or COVID, while young children are more likely to have severe flu but are at low risk for severe COVID. Even people with mild COVID can develop long-term manifestations known as long COVID.

People with a mild case of any of these illnesses can usually manage their symptoms at home with supportive care, including drinking plenty of fluids, getting enough rest and taking over-the-counter medications. Seek medical care if you or your child develop a high fever, have difficulty breathing or experience other worrisome symptoms. People at risk for more severe COVID can be treated with Paxlovid or monoclonal antibodies.

Whichever bug you have, stay home from work or school, cover coughs and sneezes, and wear a well-fitted mask (such as a KN95 or N95) to avoid transmitting it to others. To reduce the risk of infection and severe illness, get an annual flu shot and stay up to date with COVID vaccines and boosters.