The flu can strike suddenly, causing symptoms ranging from fever and muscle aches to severe fatigue.
Symptoms often change as the illness progresses, and some warning signs require particular attention.
The flu often comes on suddenly
Unlike a common cold, the flu can develop within just a few hours.
Many people experience a rapid worsening of symptoms, including fever, headaches, muscle aches and exhaustion.
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Fever is not always a symptom
Fever and chills are common signs of the flu.
However, it is important to remember that you can have the flu without developing a fever.
Your muscles may ache
Body aches and pains are among the most common flu symptoms.
Muscle aches can occur alongside fever, headaches and a general feeling of weakness.
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Fatigue can be overwhelming
The flu can cause severe fatigue, making it difficult to carry out normal daily activities.
Exhaustion may persist even after other symptoms have started to improve.
Coughing and a sore throat
A dry cough and throat irritation are common symptoms.
Some people also experience hoarseness, and the cough may last longer than the fever.
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Stomach problems can also occur
Although the flu is primarily associated with respiratory symptoms, it can also cause vomiting and diarrhoea.
These symptoms are more common in children than in adults.
Symptoms usually appear after two days
It typically takes around two days from infection for the first symptoms to appear.
However, this period can range from one to four days.
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The first three days are often the worst
During the early stages of the illness, many people experience fever, chills, headaches, muscle aches and coughing.
Symptoms are typically most intense during this period, when the risk of spreading the virus is also highest.
Symptoms may change on the fourth day
After a few days, fever and muscle aches often begin to subside.
However, coughing, throat irritation and fatigue may become more noticeable.
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Most people feel better within a week
Between the fifth and seventh days of illness, most people begin to experience a gradual improvement.
If a fever returns after disappearing or breathing becomes more difficult, medical attention should be sought.
A cough can last for several weeks
Even after recovering from the worst of the flu, coughing and fatigue may linger.
For some people, these symptoms can last two weeks or longer.
Influenza A and B cause similar symptoms
Both types of influenza can cause serious illness.
Symptoms alone cannot reliably determine which type of influenza a person has contracted. An influenza test is needed to identify the type.
Influenza A often dominates the season
Influenza A includes subtypes such as H1N1 and H3N2.
This type accounts for most seasonal flu cases and is often most widespread at the beginning and peak of the flu season.
Influenza B often appears later in the season
Influenza B typically circulates in late winter and spring.
The virus also affects children and teenagers, but like influenza A, it can cause serious illness.
The flu can be mistaken for COVID-19
The two illnesses can cause many of the same symptoms.
It is therefore impossible to reliably distinguish between them based on symptoms alone. Testing can help determine the cause.
The first 48 hours can be crucial
Prescription antiviral medications for influenza are most effective when treatment begins within 48 hours of the first symptoms appearing.
These medications can shorten the duration of the illness and reduce the risk of complications.
Some people are at greater risk
Adults over 65, children under five and pregnant women have an increased risk of complications.
This also applies to people with conditions such as asthma, heart disease, diabetes, kidney problems or weakened immune systems.
These symptoms require emergency medical attention
Difficulty breathing, persistent chest pain or pressure, confusion, seizures and severe dehydration are warning signs.
In children, additional warning signs include bluish lips, laboured breathing and reduced responsiveness.
You can spread the flu before symptoms begin
The influenza virus can spread approximately one day before an infected person notices any signs of illness.
The risk of transmitting the virus to others is typically highest during the first few days of illness.
Wait before returning to work
According to the US Centers for Disease Control and Prevention (CDC), people should stay home until their symptoms have been improving overall for at least 24 hours and they have been fever-free without using fever-reducing medication.
During the following five days, additional precautions are recommended, including good hand hygiene and keeping a distance from people at increased risk.
The flu vaccine has been updated for the season
The influenza vaccine for the 2026–2027 flu season has been updated.
It protects against three influenza viruses: H1N1, H3N2 and B/Victoria. The update reflects, among other factors, changes in the influenza viruses that circulated during the previous season.
This article is made and published by Ida-Marie Palm Varbæk, who may have used AI in the preparation.