Quick answer: the flu comes on suddenly. Fever or chills, body aches, headache, deep tiredness, a dry cough, and a sore throat often appear within hours, about 2 days after exposure. The worst of it usually lasts 3 to 4 days, and most healthy people are clearly improving within a week. Flu A and flu B feel the same, and symptoms alone cannot separate the flu from COVID-19, so a same-day swab test is the fastest way to know and to start antiviral treatment in time.
Where the 2026 to 2027 flu season stands
As of the CDC surveillance report for the week ending September 26, 2026, seasonal influenza activity in the United States was low but increasing, with 3.4% of respiratory specimens tested at clinical laboratories positive for influenza. Nearly all of the early detections were influenza A (96.3%), and among the influenza A viruses that public health laboratories subtyped, H1N1 made up 96.4%. That mix can change as the season goes on. In Southern California, flu activity typically rises in late fall and peaks between December and February.
All three components of the 2026 to 2027 flu vaccine were updated, in part because an influenza A (H3N2) subclade spread widely last season, and all flu vaccines for this season are trivalent, meaning they protect against three viruses: one H1N1, one H3N2, and one B/Victoria virus. Our flu season 2026 to 2027 guide covers timing your flu shot.
Flu symptoms to watch for
According to the CDC, people with the flu often have some or all of these:
- Fever, or feeling feverish with chills (not everyone with the flu has a fever)
- Cough, usually dry
- Sore throat
- Runny or stuffy nose
- Muscle or body aches
- Headache
- Fatigue that can be severe
- Vomiting and diarrhea in some people, more often in children than in adults
The feature that most separates the flu from a cold is speed. People often remember the hour they got sick.
Flu symptoms day by day
- Exposure to first symptoms: about 2 days, with a range of about 1 to 4 days. You can spread the virus starting about a day before you feel sick.
- Days 1 to 3: sudden fever or chills, body aches, headache, weakness, dry cough, and sore throat. This is when you feel worst and when you are most contagious.
- Day 4: fever and aches start to ease. Cough, a hoarse or sore throat, and tiredness become more noticeable.
- Days 5 to 7: steady improvement for most people. A fever that goes away and then returns, or breathing that gets worse, is a reason to be seen.
- Week 2 and beyond: cough and fatigue can linger for 2 weeks or longer even after an ordinary case.
Flu A vs. flu B: is there a difference in symptoms?
Influenza A and influenza B cause the same illness, and no symptom reliably tells them apart. The differences are in how the viruses behave across a season:
- Influenza A (the H1N1 and H3N2 subtypes) causes most seasonal flu and usually drives the early and peak parts of the season.
- Influenza B (the B/Victoria lineage) often circulates later, in late winter and spring, and is common in children and teenagers.
Either type can cause severe illness, and the same antiviral medicines treat both. A flu test reports which type you have.
Flu, COVID-19, or a cold?
- Cold: gradual start, mostly sneezing, runny nose, and a scratchy throat. Fever and body aches are mild or absent.
- Flu: abrupt start with fever, body aches, headache, and exhaustion.
- COVID-19: overlaps with both. See our guide to COVID symptoms in 2026.
- Strep throat: a severe sore throat and fever, usually without a cough or runny nose. Our strep throat vs. flu guide explains the difference.
Because the flu and COVID-19 look so similar, testing is the only dependable way to tell them apart. We test for both on site at our cold and flu service, with results while you wait.
When to get tested, and why the first 48 hours matter
Prescription antiviral medicine for the flu works best when it is started within 48 hours of the first symptoms. It can shorten the illness and lower the chance of complications. That window is the main reason not to wait several days to see whether a sudden fever goes away. Getting tested promptly matters most if you are at higher risk of flu complications:
- Adults 65 and older, and children younger than 5 (especially younger than 2)
- Pregnant patients, including up to 2 weeks after delivery
- People with asthma, chronic lung disease, heart disease, diabetes, kidney or liver disease, or a weakened immune system
Emergency warning signs
Call 911 or go to the nearest emergency room for any of these:
- Difficulty breathing or shortness of breath
- Persistent pain or pressure in the chest or abdomen
- Confusion, persistent dizziness, or trouble waking up
- Seizures
- Not urinating, or other signs of severe dehydration
- Severe muscle pain or severe weakness
- In children: fast or labored breathing, bluish lips or face, ribs pulling in with each breath, no urine for 8 hours, not alert or interacting when awake, a fever above 104 degrees Fahrenheit that does not come down with fever-reducing medicine, or any fever in an infant younger than 12 weeks
- A fever or cough that improves and then returns or worsens
For everything short of an emergency, urgent care is faster and far less expensive. Our ER or urgent care guide walks through the decision.
When can I go back to work or school?
Current CDC guidance for respiratory viruses is to stay home until, for at least 24 hours, your symptoms are improving overall and you have had no fever without fever-reducing medicine. For the next 5 days, take added precautions such as good hand hygiene, a well-fitted mask, and keeping distance from people at higher risk.
Walk in today, no appointment needed
Advanced Urgent Care of Pasadena is at 797 S Arroyo Pkwy, Pasadena, next to Petco, with free on-site parking. We are open Monday through Friday 8 AM to 8 PM and Saturday and Sunday 9 AM to 5 PM, and we take our last patient 30 minutes before closing. We serve Pasadena, South Pasadena, Altadena, San Marino, Alhambra, and nearby communities. Questions? Call (626) 304-0404.
Sources
- CDC: About Influenza
- CDC: 2026-2027 Flu Season
- CDC FluView: Weekly US Influenza Surveillance Report, Week 38, ending September 26, 2026
- CDC: Preventing Spread of Respiratory Viruses When You Are Sick
This article is general health information and is not a substitute for an examination by a medical provider. If you think you are having a medical emergency, call 911.