Flu symptoms are caused by your own immune system fighting the influenza virus, not by the virus itself directly destroying tissue. When influenza infects the cells lining your airways, your body launches an aggressive inflammatory response that produces fever, muscle aches, fatigue, and respiratory symptoms. The virus does cause some direct damage to your lungs and airways, but most of what makes you feel terrible is friendly fire from your immune defenses.
How the Virus Gets In
Influenza is remarkably efficient at reaching your airway cells. The virus uses two surface proteins to navigate your respiratory tract. The first acts like a grappling hook, latching onto sugar molecules on the surface of your airway cells. The second works like a pair of molecular scissors, snipping through the thick mucus layer that normally traps invaders. Together, these proteins create a “burnt bridge” effect: the virus cuts its way forward through mucus while being unable to drift backward, essentially motoring toward cells it can infect.
Once the virus reaches an airway cell, it tricks the cell into pulling it inside through a process called endocytosis. The virus then hijacks the cell’s machinery to make copies of itself. Each infected cell can release thousands of new virus particles before it dies, spreading the infection deeper into your respiratory tract. This chain reaction happens fast. Symptoms typically appear one to four days after exposure, and for most people, the worst hits within the first two to three days of illness.
Why You Get a Fever
Fever is your body’s deliberate decision to raise its thermostat. When immune cells detect the influenza virus, they release signaling molecules called cytokines, particularly one called IL-1 beta. These cytokines travel through your bloodstream to a region at the base of your brain that controls body temperature. There, they trigger the production of a chemical messenger called prostaglandin E2, which binds to specific receptors on your brain’s temperature-regulating neurons.
Those neurons respond by resetting your body’s target temperature upward, sometimes to 103°F or higher. Your body then works to reach that new set point: blood vessels constrict to conserve heat, you start shivering to generate warmth, and you feel intensely cold even though your temperature is rising. This is why chills and fever go hand in hand. The elevated temperature actually helps your immune system work more efficiently and makes the environment less hospitable for the virus, but it comes at a significant energy cost that leaves you drained.
This is also why fever reducers work the way they do. They block the production of prostaglandin E2, which resets the thermostat back to normal. Your temperature drops, but the underlying immune battle continues.
What Causes Muscle Aches and Fatigue
The full-body soreness that comes with the flu feels like you ran a marathon, and the explanation is surprisingly similar. Your immune system floods the bloodstream with inflammatory cytokines, including IL-6, TNF-alpha, and IL-1 beta. These molecules circulate throughout your entire body, not just your respiratory tract, which is why the flu feels so systemic compared to a common cold.
Research from the University of Connecticut has shown that influenza triggers muscle-wasting gene activity in skeletal muscle even though the virus doesn’t directly infect muscle tissue. Instead, immune cells, primarily a type of white blood cell called T cells, infiltrate muscle tissue and cause inflammation that leads to muscle breakdown and impaired function. In animal studies, this immune-driven muscle damage was more prolonged and severe in older subjects, which helps explain why the flu can be so debilitating for older adults.
The profound fatigue has a similar origin. Your body is diverting enormous metabolic resources toward the immune response. The same cytokines that cause fever also act on your brain to promote sleepiness and reduce appetite, essentially forcing you to rest so your body can focus on fighting the infection.
How the Virus Damages Your Airways
While most flu symptoms come from the immune response, the virus does directly destroy tissue in your respiratory tract. As the virus replicates, it kills the epithelial cells lining your nose, throat, and lungs through a combination of cell death pathways. This strips away the protective barrier of your airways, which is why you develop a sore throat, cough, and congestion.
In the lungs, this damage can be more serious. Destruction of the cells lining the tiny air sacs disrupts the barrier that keeps fluid out of the lungs, leading to swelling that impairs oxygen exchange. This is the mechanism behind influenza pneumonia, and it’s driven by both direct viral damage and the collateral damage from an overly aggressive immune response. In severe cases, the immune system can spiral into what researchers describe as a “cytokine storm,” where overwhelmingly high levels of IL-6 and other inflammatory molecules cause widespread tissue damage that becomes more dangerous than the virus itself.
Why Flu Feels Different From a Cold
Colds and flu both infect the respiratory tract, but they produce different symptom patterns because of the scale of the immune response they trigger. Cold viruses tend to stay in the upper airways (nose and throat) and provoke a relatively mild, localized inflammatory response. That’s why cold symptoms are mostly nasal: runny nose, sneezing, mild sore throat.
Influenza, by contrast, can infect cells deeper into the lungs and triggers a much larger systemic cytokine release. This is why flu symptoms hit more abruptly and include the whole-body misery of high fever, severe muscle aches, and deep exhaustion that colds rarely cause. The CDC defines flu-like illness simply as fever plus cough or sore throat, but in practice, the speed and intensity of onset is the biggest tip-off. A cold builds gradually over a day or two. The flu often announces itself within hours, going from fine to flattened in an afternoon.
The Symptom Timeline
Symptoms begin one to four days after exposure. The first day or two of illness is typically the worst, with high fever, intense body aches, headache, and dry cough arriving together. By days three to five, fever usually starts to break, but cough and fatigue often linger. Most previously healthy adults recover within about a week without antiviral treatment, though cough and a general feeling of being run-down can persist for two weeks or longer, especially in older adults.
The lingering fatigue and cough after the acute phase reflect the time your body needs to repair damaged airway tissue and wind down the inflammatory response. Even after the virus is cleared, the immune system doesn’t switch off instantly, and the epithelial cells lining your airways need time to regenerate.
When Symptoms Signal Something Worse
The flu can damage your airway lining enough to open the door to bacterial infections, most commonly bacterial pneumonia. This typically shows up as a second wave of illness: you start to improve, then suddenly feel worse again after a few days. Warning signs include a new or higher fever after initial improvement, cough producing yellow, green, or bloody mucus, chest pain when breathing or coughing, rapid breathing or shortness of breath, and confusion. Bluish color in the lips or nails signals dangerously low oxygen and requires emergency care.
Which Strains Are Circulating Now
During the 2024-2025 season, influenza A viruses dominated, with the H1N1 and H3N2 subtypes circulating at roughly equal levels. Influenza B activity remained low throughout the season, and all detected B viruses belonged to the Victoria lineage. The Yamagata lineage of influenza B has not been detected for five consecutive seasons and is no longer included in newer vaccine formulations. Despite the mix of strains, no unusual symptom patterns were reported. The core experience of flu, regardless of which strain causes it, remains the same: sudden onset of fever, body aches, cough, and exhaustion driven by your body’s inflammatory battle against the virus.