What Are the Early Symptoms of the Flu?
Key Takeaways
- Flu symptoms often come on suddenly. You may feel relatively normal and then quickly develop fatigue, body aches, fever, chills, or headaches.
- Early flu symptoms usually begin one to four days after infection, often around two days after exposure.
- Flu, COVID-19, and other respiratory infections can cause similar symptoms, which is why testing and consulting a clinician is important.
- If you have trouble breathing, chest pain, or extreme fever, seek medical attention.
A sore throat that slowly turns into congestion is usually a warning of a cold on its way. But when body aches, fever, or extreme fatigue come on suddenly, these may indicate early flu symptoms. Infectious disease expert Angela Branche, MD, from University of Rochester Medicine shares what signs to look out for this flu season.
What Are the First Signs of the Flu?
The first signs of the flu often affect your whole body, not just your nose and throat like respiratory illnesses. Symptoms can vary, and you may experience several at once.
Flu season has started early this year. The Centers for Disease Control and Prevention (CDC) reports that flu activity across the United States is increasing earlier than typically seen at this time of year. If you haven’t had your flu shot yet, make plans to get vaccinated as soon as possible. The vaccine may not prevent every flu infection, but it can help reduce your risk of serious illness and complications.
Common early flu symptoms include:
- Body aches and muscle pain
- Fatigue or extreme tiredness
- Fever and chills
- Headache
- Cough
- Sore throat
- Runny or stuffy nose
Does the Flu Come On Suddenly?
Yes. The flu usually has a more sudden onset than the common cold. With the flu, you might feel fine in the morning but experience fatigue, muscle aches, chills, or fever later that day. The common cold more often starts gradually, with symptoms such as a sore throat, sneezing, or a runny nose becoming noticeable over time.
How Long After Exposure Do Flu Symptoms Appear?
Flu symptoms usually appear one to four days after you become infected, but the exact timing varies. This period between when you are exposed to influenza and the beginning of flu symptoms is called the incubation period.
Think You Have the Flu?
If you're unsure whether your symptoms are influenza or need help deciding what to do next, University of Rochester Medicine can help you find the right level of care.
Can You Spread the Flu Before Symptoms Start?
Yes. You can spread influenza before you start feeling sick, usually about one day before symptoms develop. If you have the flu, you are usually most contagious during the first three days of illness. If you feel sick, stay home and limit close contact with others.
What Do Flu Symptoms Look Like Day by Day?
Flu symptoms often develop quickly, become more intense during the first few days, and then gradually improve over five to seven days.
This timeline describes a general pattern, but there is no exact day-by-day timeline that applies to everyone. Your age, overall health, and the severity of your illness can affect how long recovery takes.
Days 1-2: Symptoms Begin
Early flu symptoms may come on suddenly. You might develop several symptoms within hours, including:
- Fever or chills
- Muscle and body aches
- Headache
- Extreme tiredness
- Cough or sore throat
If you think you have the flu, consider contacting a healthcare professional early. Antiviral medications work best when started within the first 48 hours after symptoms begin.
Don't want to leave the house? Take an at-home flu or COVID-19 test and schedule a Virtual Urgent Care visit—our experts can help connect you to the right care you need based on your symptoms.
Days 3–4: Symptoms May Be at Their Worst
Many people feel the worst during the first several days of illness. Fever, body aches, headaches, and fatigue may continue, and a cough could become more noticeable.
You might still feel significantly unwell and need plenty of rest. Drink fluids regularly to help prevent dehydration.
If your symptoms are severe, continue to worsen, or include trouble breathing or chest pain, seek medical attention.
Days 5–7: Symptoms Often Begin Improving
Most people begin to gradually feel better during the second half of the first week. But you may still feel tired or have a persistent cough, even if other symptoms have improved.
If your fever returns after improving, your symptoms become worse, or you develop new symptoms during this time, contact a healthcare professional. These changes can signal a complication or another infection.
After the First Week: Can Flu Symptoms Linger?
Yes. Cough and fatigue can continue for one to two weeks or longer, even after the most intense flu symptoms have passed.
Most people recover from the flu within a few days to two weeks. However, recovery varies, and some people experience complications that require medical care.
It’s important to pay attention to whether your symptoms are improving overall. If you’re getting worse instead of better, contact a healthcare professional.
What Should You Do When Flu Symptoms Start?
If you develop flu symptoms, taking a few practical steps can help you manage your illness and reduce the risk of spreading it to others.
- Rest and drink fluids.
- Stay home while you’re sick.
- Consider testing.
- Ask about antiviral treatment early.
- Seek emergency medical care if you have trouble breathing, chest pain, confusion, severe weakness, or rapidly worsening symptoms.
Is it COVID, Flu, or RSV?
Illnesses like flu, RSV, COVID-19, and stomach bugs are more common in the fall and winter months. Stay updated on flu, RSV, and other seasonal illnesses: symptoms, prevention tips, and when to seek care.