What causes flu vs covid?
Last updated September 6, 2026.
**Flu is caused by influenza viruses (mainly influenza A and B), while COVID-19 is caused by a coronavirus called SARS-CoV-2.** They are unrelated viruses, but both spread through respiratory droplets and small particles released when an infected person breathes, talks, coughs, or sneezes. Symptoms overlap so much that you cannot reliably tell them apart without a test, though COVID-19 more often causes a loss of taste or smell and flu tends to come on abruptly with fever and body aches. COVID-19 also tends to have a longer window between exposure and symptoms, so your timeline after a known contact offers a clue rather than an answer. Testing is the only way to know which one you have, and it matters because the antiviral treatments are different. Annual flu vaccination and staying current on COVID-19 vaccination lower your chances of getting seriously ill from either.
What to do
- Test to know: Symptoms overlap too much to guess, so use a test when the answer would change your treatment or your plans around others.
- Start antivirals early: Both flu and COVID-19 antivirals work best within the first day or two of symptoms, so reach out quickly instead of waiting it out.
- Ask about your risk: If you are over 65, pregnant, or have a condition like asthma, diabetes, or a weakened immune system, tell a clinician early since treatment thresholds are lower for you.
- Vaccinate for both: Get a flu shot each fall and stay current on COVID-19 vaccination; you can usually get them at the same visit.
- Stay home while contagious: Keep away from others until your symptoms are improving and you have been fever-free for 24 hours without fever-reducing medicine.
- Know the urgent signs: Seek care right away for trouble breathing, chest pressure, bluish lips, confusion, or dehydration.
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