The flu hits fast. Unlike a cold that builds gradually, influenza typically announces itself with a sudden wave of fever, chills, body aches, and exhaustion, often within hours. Most people also develop a cough, sore throat, and a runny or stuffy nose. The full list of common symptoms includes:
- Fever or feeling feverish with chills (though not everyone with the flu runs a fever)
- Cough
- Sore throat
- Runny or stuffy nose
- Muscle or body aches
- Headache
- Fatigue
- Vomiting and diarrhea (more common in children than adults)
How Flu Symptoms Progress Day by Day
Symptoms typically show up about two days after you’re exposed, though the incubation window ranges from one to four days. The onset is abrupt. You might feel fine in the morning and be flat on the couch by the afternoon with a high fever, pounding headache, and aching muscles.
For most otherwise healthy people, the worst of it resolves within three to seven days. Fever and body aches tend to break first. The cough and a lingering sense of tiredness, though, can hang around for two weeks or longer, especially in older adults and people with chronic lung conditions. That persistent fatigue catches a lot of people off guard. You may feel well enough to return to work but still feel drained for days afterward.
How the Flu Feels Different From a Cold
Both the flu and the common cold are respiratory illnesses, but they feel meaningfully different. A cold tends to come on slowly, starting with a scratchy throat and progressing to congestion over a day or two. The flu arrives all at once, with full-body symptoms like aches, chills, and fatigue dominating the picture.
Congestion is usually the main event with a cold. People with colds are more likely to have a stuffy or runny nose as their primary complaint. With the flu, congestion is just one piece of a much more intense package. Fevers are far more common with the flu, and the overall severity is noticeably higher. If you feel like you’ve been “hit by a truck,” it’s more likely flu than a cold.
Flu vs. COVID-19 Symptoms
You cannot reliably tell the flu apart from COVID-19 based on symptoms alone. The two infections share nearly identical symptom lists: fever, cough, fatigue, sore throat, body aches, and congestion. Testing is the only way to confirm which one you have.
There are a few subtle differences. A change in or loss of taste or smell is more frequently linked to COVID-19, though it can occur with other respiratory infections. COVID-19 also tends to have a longer incubation period, with symptoms appearing two to five days after exposure (and sometimes up to 14 days), compared to the flu’s one to four days. The complications differ too: COVID-19 carries a higher risk of blood clots and can lead to long-term symptoms, while the flu is more likely to cause secondary bacterial infections like pneumonia.
Symptoms in Children
Children get many of the same symptoms adults do, but stomach problems are far more common in kids. Nausea, vomiting, abdominal pain, and diarrhea frequently accompany the fever and cough in pediatric flu cases, which is why parents sometimes mistake the flu for a stomach bug. In adults, gastrointestinal symptoms are relatively uncommon.
Young children also tend to run higher fevers and may be contagious for a longer stretch than adults. A child who seems to be recovering and then spikes a new fever deserves close attention, as this can signal a secondary infection.
Why Symptoms Can Look Different in Older Adults
Adults over 65 sometimes present with less obvious symptoms. Fever may be lower than expected or absent entirely, which can make the flu harder to recognize. Fatigue, confusion, or a general sense of feeling unwell may be the most prominent signs. Because older adults are at higher risk for complications like pneumonia, a milder-looking illness can still become serious. The lingering cough and exhaustion that follow the acute phase also tend to last longer in this age group.
When You’re Contagious
You can spread the flu before you even know you’re sick. Viral shedding begins about one day before symptoms appear and continues for five to seven days after you start feeling ill. The most contagious window is the first three days of symptoms, which is when your body is producing and releasing the most virus.
Some people, particularly young children and those with weakened immune systems, stay contagious beyond that seven-day window. It’s also possible to carry and spread the virus without ever developing symptoms yourself.
Signs of Complications
Most flu cases are unpleasant but uncomplicated. The situation that warrants real concern is when the illness seems to improve and then gets worse again. A returning fever after a few days of feeling better, increasing shortness of breath, rapid breathing, or a bluish tint to the lips or face can signal that a secondary infection like pneumonia has developed. Pneumonia following the flu is a bacterial infection that settles into the lungs and requires its own treatment.
Chest pain or pressure, persistent dizziness, and severe or worsening dehydration (especially in children who can’t keep fluids down) are also signs that the flu has moved beyond what your body can handle on its own. These complications are more common in adults over 65, young children, pregnant women, and people with chronic health conditions like asthma, diabetes, or heart disease.