Unexplained symptoms are worth exploring, but it's important to first speak to your GP and have potential medical causes appropriately investigated. This library is for information only and does not replace medical advice, diagnosis or testing.
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Flu-like symptoms describe a collection of sensations that resemble having influenza or another viral illness, even when no infection has been identified.
People may experience body aches, chills, headaches, exhaustion, temperature sensitivity, sore throats or a general feeling of being physically unwell.
The medical term malaise describes this broader sense of discomfort, illness or lack of wellbeing. It is often difficult to pinpoint to one particular part of the body.
For some people, it feels like the early stages of a virus, when the body begins to ache and a sense of illness develops before more recognisable symptoms appear.
For others, the feeling can be persistent, returning repeatedly or fluctuating without developing into an identifiable infection.
Flu-like symptoms and malaise are reported in conditions including ME/CFS, Long COVID, fibromyalgia, autoimmune and inflammatory conditions, as well as during infections.
They may occur independently or alongside other symptoms such as fatigue, pain, cognitive dysfunction and post-exertional malaise (PEM).
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The sensation of being unwell involves interactions between the immune system, nervous system and brain.
During an infection, the body produces a coordinated response that can influence temperature, energy levels, pain sensitivity, appetite and behaviour.
Some of these same sensations can also occur in non-infectious illnesses, although the underlying processes may be different.
The Immune System and Sickness Behaviour
When the immune system responds to infection or inflammation, it releases signalling molecules, including substances known as cytokines.
These molecules help coordinate the immune response and can also communicate with the brain.
This interaction contributes to what researchers call sickness behaviour: a collection of responses associated with being ill.
These can include:
Fatigue and reduced energy.
Increased sensitivity to pain.
Reduced appetite.
Changes in sleep and alertness.
Difficulty concentrating.
A general feeling of being unwell.
These responses help explain why an infection can affect the whole body rather than just the area where it began.
However, experiencing these sensations does not necessarily mean an infection or an abnormal immune response is present.
Why the Whole Body Can Ache
During an illness, immune signalling and inflammatory processes can influence pain sensitivity.
This may contribute to the widespread muscle and joint discomfort commonly associated with influenza and other viral infections.
The nervous system also plays a role in how these sensations are experienced.
In some chronic conditions, people report similar widespread aches or tenderness without an identifiable acute infection.
The mechanisms behind these symptoms can vary between conditions and are not always fully understood.
Feeling Feverish Without a Fever
A common feature of flu-like malaise is feeling feverish despite having a normal measured body temperature.
Someone may experience chills, shivering, warmth or a sensation of internal heat without an elevated thermometer reading.
The experience of temperature involves information from the skin, circulation, nervous system and brain, not simply core body temperature.
As a result, the sensation of being feverish does not always correspond to a measurable fever.
The Sense of Being Unwell
Malaise is different from a single symptom such as pain or fatigue.
It describes an overall feeling that something is physically wrong or that the body is unwell.
The brain continuously receives information about the body's internal state, a process known as interoception.
This includes signals related to temperature, inflammation, circulation, digestion and other bodily functions.
These signals contribute to our overall sense of physical wellbeing.
When this internal experience changes, someone may feel distinctly ill even if they struggle to identify or describe a specific sensation.
Flu-Like Symptoms and Post-Exertional Malaise
Flu-like symptoms can form part of post-exertional malaise (PEM), particularly in people with ME/CFS and some people with Long COVID.
PEM describes a worsening of symptoms following physical, cognitive, emotional or other exertion.
For some people, this worsening feels remarkably similar to developing a viral illness, with increased body aches, chills, sore throat, headache and profound exhaustion.
However, flu-like symptoms and PEM are not interchangeable.
Flu-like malaise can occur independently of exertion, while PEM can involve a much broader range of symptoms.
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Flu-like symptoms don't always mean an infection is present.
Similar sensations can occur in inflammatory conditions, chronic illnesses and other health problems. However, a new infection should not be ruled out simply because someone regularly experiences these symptoms.
Malaise is more than tiredness.
Fatigue describes exhaustion, while malaise refers to a broader feeling of being physically unwell. The two often overlap but are not identical.
You can feel feverish without having a fever.
Chills, warmth and shivering may occur despite a normal thermometer reading.
Symptoms can resemble the beginning of an illness.
Some people repeatedly experience the familiar feeling of coming down with something, without progressing to a recognisable infection.
Flu-like sensations can affect the whole body.
Rather than one specific area, people may experience a combination of aches, heaviness, headache, chills and exhaustion.
A sore throat can occur alongside malaise.
Some people experience intermittent throat discomfort or tenderness around the neck. These symptoms can have many causes and are not specific to any one condition.
Flu-like symptoms can overlap with PEM.
For some people, a flu-like feeling is part of a wider symptom flare following exertion, but the presence of flu-like symptoms alone does not establish PEM.
Symptoms may fluctuate without an obvious pattern.
The sensation of being ill may appear suddenly, last for hours or days, or change considerably in intensity.
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Flu-like malaise can be particularly difficult to explain because it often feels like a familiar illness without following its expected course.
The coming-down-with-something feeling: Experiencing the unmistakable sensation that a virus is beginning, but without developing a recognisable infection.
The full-body ache: Feeling widespread soreness or tenderness similar to having influenza.
The feverish feeling: Experiencing chills, warmth or shivering despite a normal body temperature.
The heavy, unwell body: Feeling physically weighed down, sluggish or generally ill, beyond ordinary tiredness.
The sore throat: Experiencing intermittent throat discomfort, sometimes alongside other flu-like sensations.
The sudden wave of illness: Feeling relatively comfortable before an unexpected sense of being unwell develops.
The lingering malaise: Experiencing a background feeling of illness that continues for days or longer.
The fluctuating symptoms: Feeling distinctly unwell at one point and considerably better at another, without a clear explanation.
For some people, these sensations are occasional and relatively mild. For others, they can be intense enough to interrupt everyday activities or make it difficult to concentrate, move comfortably or interact with others.
One of the more confusing aspects is the familiarity of the feeling. Most people recognise what it feels like to develop a virus, so experiencing those same sensations repeatedly without a clear infection can be unsettling.
The combination of symptoms may also change. One episode might involve chills and body aches, while another is dominated by headache, sore throat or an overwhelming sense of illness.
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Flu-like symptoms can occur with infections, inflammatory and autoimmune conditions, medication reactions and other medical problems.
New, persistent, recurrent or worsening flu-like symptoms should be assessed by a healthcare professional, particularly when accompanied by a measured fever, swollen glands, unexplained weight loss or other new symptoms.
It is important not to assume that new flu-like symptoms are part of an existing chronic condition, as infections and other illnesses can occur independently.
Symptoms such as severe breathing difficulties, confusion, a non-fading rash, a severe headache with neck stiffness or signs of serious illness can require emergency medical attention.
