Flu symptoms usually arrive suddenly: chills, fever or feeling feverish, body aches, headache, fatigue, cough, sore throat, and a runny or stuffy nose. Flu and COVID-19 share many symptoms, so it can be hard to tell them apart from symptoms alone; testing and a clinician can help sort it out when the answer affects your care.
As of October 2026, the practical difference is not which symptom chart looks most familiar, but whether you are getting worse, have higher-risk circumstances, or may benefit from prompt testing and clinician-directed treatment.
What flu symptoms should you look for?
MedlinePlus describes influenza as a viral respiratory illness. Chills are a common flu symptom, and a person can have flu without a measured fever.
| Symptom | How it can fit flu | What to remember |
|---|---|---|
| Chills or feverish feeling | Common and often sudden | Not everyone with flu has a fever. |
| Body aches and headache | Common | Body aches are a common flu symptom. |
| Cough, sore throat, stuffy or runny nose | Common | These overlap with other respiratory viruses. |
| Fatigue | Common | Tiredness can outlast the first acute symptoms. |
| Vomiting or diarrhea | Can occur | They are more common in children; “stomach flu” usually means gastroenteritis, not influenza. |
MedlinePlus says colds, flu, and COVID-19 are often confused because they share symptoms. It also explains that “stomach flu” usually means gastroenteritis, not influenza. Sinus infection and pneumonia are possible flu complications.
Is it a cold, flu, COVID-19, or RSV?
Flu and COVID-19 share many symptoms, and MedlinePlus says testing is the best way to distinguish them. A cold often starts more gradually and tends to be milder; flu more often starts suddenly with fever or chills and body aches.
| Illness | Pattern that may stand out | How to get clarity |
|---|---|---|
| Common cold | Symptoms often build gradually; fever is less common. | Consider the overall course and speak with a clinician if symptoms are severe or persistent. |
| Flu | Often sudden; chills, feverishness, aches, headache, fatigue and cough are common. | A clinician can arrange flu testing when it will guide care. |
| COVID-19 | Can closely resemble flu. | Use an appropriate COVID-19 test and discuss results or ongoing concern with a clinician. |
The Wisconsin Department of Health Services notes that flu symptoms can begin one to four days after exposure and that COVID-19 symptoms can look very similar. Its flu guidance also notes that influenza infects the nose, throat, and lungs.
How long does seasonal flu last?
For many people, flu symptoms improve over a few days to two weeks. MedlinePlus says fever commonly resolves in three to four days, though it can last up to eight days; cough and tiredness may remain for weeks. A return of fever after improvement, or a worse cough, deserves prompt medical attention because it can accompany complications.
MedlinePlus recommends getting plenty of rest and drinking plenty of liquids. The key is to stay home and away from others while ill, and follow your clinician’s advice for your situation.
What actually helps when flu symptoms start?
There is no one route for everyone. The choices below have different purposes, and early contact with a clinician matters most for people at higher risk or people who are very sick.
| Option | What it is for | Who it may suit | Cost | Limits |
|---|---|---|---|---|
| Rest, fluids, and symptom comfort | Comfort while your illness runs its course | People with mild symptoms, guided by their own care needs | Not provided in the evidence | Does not identify which virus is causing symptoms. |
| Nonprescription fever/ache relief | Relief of fever and aches | People for whom a clinician or pharmacist says an option is appropriate | Not provided in the evidence | Check active ingredients and age restrictions; aspirin should not be used for flu in children or teens. |
| Flu antiviral medicine | Clinician-directed flu treatment | People who are very sick or at higher risk of flu complications | Not provided in the evidence | MedlinePlus says these work best when started within two days of symptom onset. |
| Testing and clinical assessment | Separating flu from COVID-19 and other respiratory infections | People for whom a clinician determines testing will guide treatment. | Not provided in the evidence | A symptom chart alone cannot make the distinction. |
MedlinePlus says antibiotics do not treat flu because influenza is caused by viruses. Ask a clinician or pharmacist before combining cold-and-flu products, especially if you take other medicines.
Who should contact a clinician promptly?
People age 65 or older, children younger than 5, pregnant people, people living in long-term care, and people with certain chronic conditions or a weakened immune system have a higher chance of serious flu complications.
Get emergency help for difficulty breathing or shortness of breath, chest or abdominal pain or pressure, sudden dizziness, confusion, severe vomiting, or flu-like symptoms that improve and then return with fever and a worse cough. For children and infants, fast or troubled breathing, bluish skin, poor drinking, lack of interaction, no tears when crying, or far fewer wet diapers are emergency signs listed by the Wisconsin Department of Health Services.
What about cancer and flu-like symptoms?
Bring a persistent, unexplained, severe, or recurring symptom pattern to a clinician rather than trying to match it to a single illness online.
Can supplements replace flu treatment or prevention?
Readers considering probiotics can review probiotic use in immunocompromised patients and probiotic-drug interactions.
Before you decide what to do next
- Write down when symptoms started, whether they began suddenly, and any fever, chills, cough, breathing change, or gastrointestinal symptoms.
- Ask a clinician whether testing for flu, COVID-19, or another infection would change your care.
- Ask whether you are in a higher-risk group and whether time-sensitive antiviral treatment is appropriate.
- Before using an over-the-counter combination product, ask a pharmacist to check for duplicate active ingredients and medicine interactions.
This article is for general information purposes only and does not constitute medical advice. Consult your doctor or qualified healthcare provider before making changes to your health routine.