CardivoCardivo guides
Flu symptoms: signs, how long the flu lasts and when to worry
Key takeaways
- Flu symptoms usually start suddenly: fever or chills, cough, sore throat, runny nose, body aches, headache and fatigue.
- Fever typically lasts 3–4 days; most people recover in a few days to under two weeks.
- Antiviral medicines work best when started within 48 hours of symptoms.
- Flu raises the short-term risk of a heart attack about sixfold in the week after infection.
Common flu symptoms
- Fever or feeling feverish, chills (not everyone with flu has a fever)
- Cough
- Sore throat
- Runny or stuffy nose
- Muscle or body aches
- Headache
- Fatigue — often severe
- Vomiting and diarrhea, more common in children
Symptoms list from the CDC. Not sure if it's a cold or COVID? See flu vs. cold vs. COVID.
Flu timeline: how long does the flu last?
| Stage | Typical timing |
|---|---|
| Exposure to first symptoms | About 1–4 days (average 2) |
| Fever | About 3–4 days |
| Most contagious | First 3–4 days after illness starts |
| Recovery | A few days to under 2 weeks; cough and tiredness can linger |
Emergency warning signs
Adults — get medical care right away for:
- Trouble breathing or shortness of breath
- Ongoing chest or abdominal pain or pressure
- Lasting dizziness, confusion or difficulty staying awake
- Seizures, not urinating, severe muscle pain, severe weakness
- Fever or cough that improves, then returns or worsens
- Worsening of a chronic condition such as heart disease, asthma or diabetes
Children: fast or labored breathing, bluish lips or face, ribs pulling in with each breath, chest pain, dehydration (no urine for 8 hours, no tears), not alert when awake, fever above 104 °F that doesn't respond to medicine, or any fever in a baby under 12 weeks.
Who is at higher risk?
Adults 65 and older, young children, pregnant people, and anyone with heart disease, diabetes, asthma or COPD, kidney disease, or a weakened immune system. If you are in a higher-risk group, call your doctor as soon as flu symptoms start.
Treatment
- Rest and fluids. Fever reducers such as acetaminophen or ibuprofen can ease aches (follow label doses).
- Antivirals such as oseltamivir or baloxavir can shorten illness and lower the risk of complications; they work best within 48 hours.
- Stay home until you've been fever-free for 24 hours without fever-reducing medicine.
- Prevention: an annual flu vaccine is recommended for nearly everyone 6 months and older.
The flu and your heart
Flu does more than make you feel awful. Fever and inflammation raise your heart rate — often by about 10 bpm for each 1 °C of fever — and a study in the New England Journal of Medicine found the risk of heart attack was about six times higher in the week after a confirmed flu infection. Flu can also push up blood sugar in people with diabetes (see high blood sugar).
When you're sick, Cardivo lets you check your heart rate and an SpO₂ estimate from your fingertip on the iPhone camera and log temperature notes, glucose and blood pressure in one place — handy if you need to tell a doctor how things changed. A resting pulse that stays high after the fever is gone is worth mentioning. Read more about a high resting heart rate.
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
What are the first signs of the flu?
Flu usually starts suddenly with fever or chills, body aches, headache and fatigue, followed by cough and sore throat.
How long does the flu last?
Fever usually lasts 3–4 days, and most people recover within a few days to less than two weeks. Cough and tiredness can last longer.
How long is the flu contagious?
People are most contagious in the first 3–4 days after illness starts and can spread flu from about a day before symptoms begin.
Can the flu affect your heart?
Yes. Fever raises heart rate, and studies show heart attack risk is about six times higher in the week after a confirmed flu infection.
Sources
- CDC — Signs and symptoms of flu
- CDC — How flu spreads
- CDC — Flu treatment
- Kwong et al., NEJM 2018 — Acute myocardial infarction after influenza