What influenza is
The CDC describes flu as a contagious respiratory illness caused by influenza viruses that can cause mild to severe illness and, at times, death. Mayo Clinic explains that it infects the nose, throat and lungs, and that influenza viruses are different from the so-called “stomach flu” viruses that mainly cause diarrhea and vomiting.
Flu spreads through droplets when an infected person coughs, sneezes or talks, and also by touching contaminated surfaces and then the eyes, nose or mouth. According to Mayo Clinic, people can spread the virus from about a day before symptoms appear until about 5 to 7 days after they start, and children and people with weakened immune systems may be contagious for longer. These are typical figures, not a guarantee that someone is no longer contagious on a given day.
Common respiratory and whole-body symptoms
Flu symptoms usually start suddenly, about 1 to 4 days after exposure. According to the CDC and Mayo Clinic, people often have some or all of the following:
- fever or feeling feverish, and chills
- cough, which can be severe
- sore throat
- runny or stuffy nose
- muscle or body aches, often marked
- headache
- tiredness and weakness
- loss of appetite, and painful or watery eyes
- wheezing or shortness of breath in some people
Not everyone has all these symptoms. If a cough lingers after the infection, see post-viral cough.
Flu without fever and variation by age
Both sources stress that not everyone with flu has a fever, so the absence of fever does not rule flu out. According to the CDC table, when fever does occur it is common and lasts 3 to 4 days.
Symptoms can look different in children. Mayo Clinic notes that they may show up more generally as fussiness or irritability, and children are more likely than adults to have vomiting, diarrhea or an upset stomach. In adults, flu is mainly a respiratory and whole-body illness, not a stomach infection.
Timing and recovery without fixed stages
Flu does not follow a fixed set of stages that is the same for everyone. The CDC states that most people recover in a few days to less than two weeks, and Mayo Clinic says that in young, healthy people flu usually goes away in a week or two with no lasting effects, although they may feel very unwell while it lasts. Tiredness and cough may take longer to settle in some people.
Flu circulates mostly in seasons: in the Northern Hemisphere usually from October to March, sometimes until May, and in the Southern Hemisphere usually from April to September. The symptoms described here are general and do not describe any particular season’s outbreak. Symptoms that improve and then come back or get worse are a warning sign, not part of normal recovery.
Why symptoms overlap with other infections
Several respiratory infections cause similar symptoms. The CDC compares flu with the common cold: a cold usually starts gradually, fever and headache are rare, aches are slight and sneezing, a stuffy nose and a sore throat are common. Flu usually starts abruptly, with fever, marked aches, tiredness and headache being common. These are tendencies, not rules.
The CDC also notes that flu and COVID-19 are both contagious respiratory illnesses caused by different viruses. Because symptoms overlap, they cannot reliably tell flu, COVID-19 or a cold apart on their own. For the difference with allergies, see allergies vs cold.
When tests may help and their limits
The CDC notes that several tests are available to detect influenza viruses. Whether a test is useful depends on the person’s situation, for example if they are at higher risk of complications or if the result would change treatment, and a clinician should interpret results together with symptoms and context.
Most healthy people with mild symptoms can manage flu at home without seeing a clinician, according to Mayo Clinic. Testing is not a substitute for watching for warning signs.
Who should seek early clinical advice
Anyone can get seriously ill with flu, but some groups have a higher risk of complications. The CDC and Mayo Clinic include:
- adults aged 65 and older
- young children, especially those under 2 years
- people who are pregnant
- people with chronic conditions such as asthma or other lung diseases, diabetes, heart disease, neurological, kidney, liver or blood diseases
- people with a weakened immune system
- people with severe obesity
- residents of care homes and other shared facilities
Mayo Clinic advises people at risk of complications who develop flu symptoms to contact a healthcare professional right away, because antiviral medicine works best when started as soon as possible, often within two days of symptom onset. That window is when benefit is greatest, not a deadline after which help is no longer worth seeking.
Emergency warning symptoms and complications
According to the CDC and Mayo Clinic, adults should get medical care right away for:
- difficulty breathing or shortness of breath
- persistent pain or pressure in the chest or abdomen
- persistent dizziness, confusion or difficulty waking
- seizures
- not urinating or signs of dehydration
- severe muscle pain, severe weakness or unsteadiness
- fever or cough that improve and then return or worsen
- worsening of a chronic condition
In children, additional signs include fast or difficult breathing, ribs pulling in with each breath, bluish or gray lips or face, no tears when crying with a dry mouth and little urine, and not being alert. In babies younger than 12 weeks, any fever needs medical attention. The CDC adds that these lists are not complete.
Complications include sinus and ear infections, pneumonia, which can be caused by the virus itself or by a bacterial infection on top of it, inflammation of the heart, brain or muscles, and sepsis. Flu can also worsen chronic conditions such as asthma or heart disease.
Where the sources differ
The two sources agree on the main symptoms and warning signs, with differences in emphasis. The CDC page (August 2024) gives a comparison table with the common cold and lists vomiting and diarrhea as more common in children, while Mayo Clinic (August 2026) adds loss of appetite, painful or watery eyes and wheezing, and gives the incubation period and how long people are contagious. Mayo Clinic notes that a young person on long-term aspirin therapy is at risk of Reye’s syndrome if infected, so check with a clinician before giving children or teenagers any medicine containing aspirin.
Mayo Clinic’s section on vaccines describes US products and schedules for the 2026–2027 season, which may not match what is authorized or offered in other countries. Vaccination recommendations should be checked with local health services. Because flu is caused by a virus, antibiotics do not treat flu itself; a clinician may consider them only if a bacterial complication is suspected. Antiviral medicines are prescribed by a clinician based on the person’s risk and timing.
In summary
Flu usually starts suddenly with fever or chills, aches, tiredness, headache, cough and sore throat, although not everyone has a fever. Most people recover at home within a few days to two weeks, and symptoms alone cannot reliably distinguish flu from COVID-19 or a cold.
People at higher risk of complications should contact a clinician early, ideally within the first two days. Anyone with breathing difficulty, chest pain, confusion, dehydration or symptoms that improve and then worsen needs urgent care. Keeping a simple record of when symptoms started, any risk factors, temperature and fluid intake can help when talking to a clinician.
For adults with ear, nose or throat problems that persist after recovery, such as ongoing sinus symptoms or ear discomfort, our ENT practice in Bilbao can see you by appointment (consultations are in Spanish). It does not replace primary care or emergency services for acute flu.
Frequently asked questions about flu symptoms
These answers are general information and do not replace an examination or individual medical advice.
What symptoms can flu cause?
Fever or chills, cough, sore throat, runny or stuffy nose, muscle aches, headache and tiredness, usually starting suddenly. Some people also have loss of appetite or watery eyes, and children may have vomiting or diarrhea.
Can I have flu without a fever?
Yes. Both the CDC and Mayo Clinic note that not everyone with flu has a fever, so its absence does not rule flu out.
Can symptoms reliably distinguish flu from COVID-19?
No. The two illnesses share many symptoms. Testing, interpreted by a clinician, is needed to tell them apart when it matters.
How long can recovery take?
Most people recover in a few days to less than two weeks, although tiredness or cough can last longer. Symptoms that improve and then worsen need medical review.
When should I contact a clinician early?
If you are at higher risk of complications, for example older than 65, pregnant, or living with a chronic condition or weakened immune system. Antiviral treatment works best when started soon after symptoms begin.
Which symptoms need urgent medical care?
Difficulty breathing, chest pain or pressure, confusion or trouble waking, seizures, dehydration, severe weakness, or fever or cough that improve and then return or worsen.
Sources
This guide is for general information and does not replace medical advice. How a problem is diagnosed and treated depends on your examination and medical history. If you think you have an emergency, contact your local emergency services.