The flu typically hits fast and hard, with fever, body aches, cough, and exhaustion that come on suddenly, often within hours. Symptoms usually appear about two days after exposure (though the range is one to four days) and last anywhere from a few days to two weeks. Unlike a cold, which tends to build gradually, the flu announces itself all at once.
Core Flu Symptoms
The hallmark of influenza is how many systems it affects at the same time. You’re not just dealing with a stuffy nose. The most common symptoms include:
- Fever and chills: Often the first thing you notice, sometimes with sweating
- Body and muscle aches: Can be severe enough to make moving around feel like a chore
- Headache
- Dry cough
- Sore throat
- Runny or stuffy nose
- Fatigue and weakness: Often the most persistent symptom, sometimes lingering for weeks
Some people also experience vomiting and diarrhea, though this is more common in children than adults. Not everyone with the flu runs a fever, but most do, and it can spike quickly.
How Symptoms Progress Day by Day
The flu follows a fairly predictable arc. During the first three days, fever, headache, muscle pain, weakness, a dry cough, and sore throat all arrive together, often suddenly. Many people describe feeling fine in the morning and completely wiped out by the afternoon.
By around day four, the fever and muscle aches typically start to ease. What takes their place is more noticeable: a hoarse or raw throat, a worsening cough, and sometimes mild chest discomfort. You’ll likely still feel drained and flat.
By day eight, most symptoms are fading. But the cough and tiredness are the stubborn ones. They can hang around for one to two weeks or even longer, well after you otherwise feel better. This lingering fatigue catches people off guard. You may feel ready to return to your normal routine but find yourself winded or exhausted far more easily than expected.
Flu Symptoms in Children
Children get many of the same symptoms as adults, but a few things look different. Gastrointestinal symptoms like vomiting and diarrhea are significantly more common in kids. High fevers also tend to spike higher in children, and younger kids may not be able to describe what hurts, so watch for irritability, refusal to eat or drink, and unusual sleepiness.
For babies younger than 12 weeks, any fever at all is considered a warning sign that needs immediate medical attention. In older children, a fever above 104°F that doesn’t come down with fever-reducing medicine is also a red flag.
How the Flu Looks Different in Older Adults
Flu symptoms in people over 65 can be deceptively mild or unusual. Average body temperature naturally runs slightly lower in older adults, so what looks like a low-grade fever may actually signal a significant infection. The National Institute on Aging notes that a single reading above 100°F, repeated readings above 99°F, or a rise of more than 2°F above a person’s normal temperature can all indicate flu or another infection.
Older adults are also more likely to experience confusion, dizziness, and weakness as prominent symptoms, sometimes without the classic high fever and body aches that younger people get. This makes the flu easier to miss or mistake for something else in this age group.
Flu vs. Cold vs. COVID-19
A cold, the flu, and COVID-19 share enough symptoms that you genuinely can’t tell them apart by how you feel alone. Testing is the only reliable way to confirm which one you have. That said, some patterns can offer clues.
A cold usually creeps in over a day or two, centering on the nose and throat: sneezing, runny nose, mild sore throat. Fever is uncommon, and body aches are mild if present at all. The flu, by contrast, slams you with whole-body symptoms. Fever, intense muscle pain, and deep fatigue dominate, often pushing the nasal symptoms into the background.
COVID-19 overlaps heavily with both but is more commonly associated with a loss of taste or smell, something that rarely happens with the flu. COVID’s incubation period also runs longer, typically two to five days but potentially up to 14 days, compared to the flu’s one to four days. Another difference: people with COVID tend to be contagious for longer, averaging about eight days after symptoms begin, while people with the flu are most contagious during the first three days and generally stop shedding the virus within five to seven days.
When You’re Contagious
You can spread the flu starting about one day before your symptoms even appear, which is part of why it moves through households and workplaces so efficiently. You’re most contagious during the first three days of illness, and most people remain potentially infectious for five to seven days after getting sick. Young children and people with weakened immune systems may be contagious for longer.
This means you can pass the virus to someone before you even know you’re sick, and you’re still a risk to others for several days after you start feeling better.
Possible Complications
Most people recover from the flu within one to two weeks without lasting problems. But the flu can trigger serious complications, especially in young children, adults over 65, pregnant women, and people with chronic conditions like asthma, heart disease, or diabetes.
Sinus and ear infections are among the milder complications. Pneumonia is the most common serious one, and it can result from the flu virus itself or from a bacterial infection that takes hold while your immune system is occupied fighting the virus. These secondary bacterial infections are actually more common with the flu than with COVID-19.
Less common but severe complications include inflammation of the heart muscle, brain, or skeletal muscle tissue, as well as kidney failure and sepsis. People with asthma may experience asthma attacks during the flu, and those with heart disease may see their condition worsen.
Emergency Warning Signs
Certain symptoms signal that the flu has become dangerous and needs immediate medical care. In adults, these include difficulty breathing, persistent chest or abdominal pain, confusion or inability to stay alert, seizures, not urinating, severe muscle pain, and extreme weakness or unsteadiness.
In children, watch for fast or labored breathing, bluish lips or face, ribs pulling in visibly with each breath, chest pain, refusal to walk due to muscle pain, and signs of dehydration such as no urine for eight hours, dry mouth, or no tears when crying. A child who is not alert or interactive when awake needs urgent evaluation.
One pattern that applies to both adults and children: a fever or cough that seems to improve and then returns or gets worse. This rebound often signals a secondary infection like pneumonia and warrants prompt attention.