Cardivo guides

Flu vs. cold vs. COVID: how to tell the difference

Updated October 30, 2026 · Wellness education, not medical advice

Key takeaways

Flu hits suddenly with fever, chills and body aches.
A cold comes on gradually and is mostly sneezing, a stuffy nose and a sore throat.
COVID-19 can look like either; new loss of taste or smell is more typical of COVID.
Symptoms overlap a lot — only a test can tell flu and COVID apart.

Symptoms chart

Symptom	Cold	Flu	COVID-19
Onset	Gradual	Sudden	Gradual or sudden
Fever	Rare	Common, often high	Common
Body aches	Mild	Common, often severe	Common
Chills	Uncommon	Common	Sometimes
Fatigue	Mild	Common, can be extreme	Common, can linger
Sneezing	Common	Sometimes	Sometimes
Stuffy or runny nose	Common	Sometimes	Sometimes
Sore throat	Common	Sometimes	Sometimes
Cough	Mild to moderate	Common	Common
Headache	Rare	Common	Common
New loss of taste or smell	Rare	Rare	More typical
Shortness of breath	Rare	Sometimes	Sometimes

How they differ

Common cold

Caused by many viruses, most often rhinoviruses. It builds over a day or two, usually without fever in adults, and runs its course in about 7–10 days. Serious complications are uncommon.

Flu (influenza)

Comes on fast — people often remember the hour they got sick. Fever, chills, aches and exhaustion dominate. Flu can lead to pneumonia and raises short-term heart attack risk. Read the full flu symptoms guide.

COVID-19

Can range from no symptoms to severe illness. Symptoms may appear 2–14 days after exposure. Loss of taste or smell is more characteristic, though less common with recent variants. Some people develop long-lasting symptoms (long COVID).

When should you test?

If you have fever, cough or aches during flu season — especially if you are 65+, pregnant, or have heart disease, diabetes, asthma or a weak immune system — get tested early. Home combination tests can check for flu and COVID at once. Knowing which virus it is matters because flu antivirals work best within 48 hours and COVID has its own treatments.

When to get medical care

Trouble breathing or shortness of breath
Chest pain or pressure
New confusion or trouble staying awake
Bluish lips or face
Symptoms that improve and then get worse

Recover smarter

Rest, fluids, and fever reducers as directed.
Stay home until fever-free for 24 hours without medicine.
Ease back into exercise — a resting heart rate still above your normal is a sign your body is still recovering.

With Cardivo you can check your heart rate, HRV and an SpO₂ estimate with your iPhone camera while you're ill and watch them return to your baseline before you go back to hard workouts. Learn what's normal in resting heart rate and HRV explained.

This guide is general wellness education based on the public sources below. It is not medical advice, diagnosis, or treatment. Cardivo is not a medical device. If you think you are having a medical emergency, call 911.

Frequently asked questions

How can I tell if I have the flu or a cold?

Flu starts suddenly with fever, chills, body aches and exhaustion. A cold builds gradually and is mostly sneezing, a stuffy nose and a sore throat, usually without fever in adults.

How do I know if it's flu or COVID?

The symptoms overlap too much to tell reliably. A flu or COVID test — including home combination tests — is the only way to know.

Is loss of smell a sign of flu or COVID?

New loss of taste or smell is more typical of COVID-19, though it can happen with other infections.

Sources

Cardivo is an ad-free iPhone app that reads heart rate and HRV from your fingertip on the camera in about 30 seconds — no Apple Watch needed — and keeps blood pressure, blood sugar, and mood in one journal you can share with your doctor as a PDF. Download on the App Store.