Viral myositis is inflammation of the skeletal muscles triggered by a viral infection. It typically causes sudden, severe muscle pain, most often in the calves, and tends to resolve on its own within a week. While it can look alarming, especially in children who may refuse to walk, the condition is usually benign.
How Viral Infections Damage Muscle
The exact mechanism behind viral myositis isn’t fully understood, but researchers have identified two likely pathways. In some cases, the virus directly invades muscle tissue and damages muscle cells through a toxic effect. In others, the virus never reaches the muscle at all. Instead, the immune response triggered by the infection causes collateral damage to muscle fibers. Both pathways lead to the same result: inflamed, painful muscles that temporarily lose strength.
Which Viruses Cause It
Influenza A and B are the most common culprits in the United States. This is the classic scenario: a child or adult comes down with the flu, starts to feel better after a few days, then suddenly develops intense leg pain.
Many other viruses can trigger myositis, including:
- Enteroviruses such as coxsackievirus B, which also causes a condition called pleurodynia (sharp chest and abdominal pain from inflamed muscles around the ribs)
- Respiratory viruses like parainfluenza, adenovirus, and RSV
- Herpesviruses including Epstein-Barr virus, cytomegalovirus, and varicella (chickenpox)
- Hepatitis B and C
- HIV
- Dengue virus
SARS-CoV-2 has joined this list. A retrospective study at a hospital in Taiwan, covering 2018 through 2024, found that COVID-19 accounted for about 17% of childhood myositis cases, while influenza was responsible for roughly 43%.
What It Feels Like
The hallmark of viral myositis is sudden, intense pain in the calf muscles on both sides. It typically hits about three days after the initial viral illness starts to resolve. You or your child may feel like the worst of the cold or flu is over, and then wake up with legs so sore that walking becomes difficult or impossible.
The pain is concentrated in the calves (specifically the gastrocnemius and soleus muscles), though it can spread to the front of the thighs. Upper body and trunk involvement is rare. The feet may rest in a slightly pointed-down position, and pulling the toes upward stretches the inflamed calf muscles and makes the pain worse. Neurological function stays normal: reflexes, sensation, and actual nerve-driven muscle power are all intact, even though the muscles hurt too much to use comfortably.
Viral Myositis in Children
The pediatric form, called benign acute childhood myositis (BACM), is the most well-studied presentation. It overwhelmingly affects boys between the ages of 3 and 14. The pattern is remarkably consistent: upper respiratory symptoms like fever, cough, sore throat, and runny nose come first. Then, often after a night of sleep, the child wakes up with bilateral calf pain and either limps heavily or refuses to walk entirely.
For parents, this can be frightening. A child who seemed to be recovering from a routine illness suddenly can’t stand up. But the condition is self-limiting. With basic supportive care, all clinical symptoms typically resolve within one week, and some children bounce back within 24 hours.
How It’s Diagnosed
Doctors diagnose viral myositis based on the clinical picture (bilateral calf pain following a viral illness) combined with a blood test measuring creatine kinase, or CK. This enzyme spills out of damaged muscle cells into the bloodstream, so elevated levels confirm that real muscle injury is happening rather than just general soreness.
In straightforward viral myositis, CK levels are elevated but moderate. When muscle breakdown becomes severe, a condition called rhabdomyolysis, CK levels can climb to five times the upper limit of normal or higher, sometimes reaching extremely high values. At that point the concern shifts to kidney damage, because the proteins released from destroyed muscle cells can clog the kidneys’ filtering system.
A normal neurological exam helps distinguish viral myositis from more serious conditions like Guillain-Barré syndrome, which also causes leg weakness after viral illness but involves nerve damage rather than muscle inflammation.
Treatment and Recovery
Benign viral myositis requires only supportive care. That means rest, adequate fluids, and over-the-counter pain relief. Staying well-hydrated is particularly important because it helps the kidneys clear any muscle proteins from the bloodstream and reduces the risk of kidney complications.
The immunosuppressive treatments used for chronic inflammatory myositis (autoimmune conditions where the body attacks its own muscle tissue over months or years) are not appropriate for the acute viral form. Those therapies, which involve high-dose steroids and other immune-suppressing medications, target a fundamentally different disease process. Viral myositis resolves because the underlying infection clears, and the muscle inflammation goes with it.
Most people recover fully within a week. In many cases, especially in children, the pain improves dramatically within one to three days. CK levels drop quickly once the acute phase passes, and muscle strength returns to normal. Permanent muscle damage from a single episode of benign viral myositis is not expected.
When It Becomes More Serious
The main complication to watch for is rhabdomyolysis, where muscle breakdown is severe enough to release dangerous amounts of protein into the blood. Signs include dark brown or tea-colored urine, dramatically reduced urine output, and worsening pain that spreads beyond the calves. This requires hospital-level care with aggressive intravenous fluids to protect the kidneys.
Rhabdomyolysis from viral myositis is uncommon but not rare. It has been documented with influenza, COVID-19, and several other viruses. The transition from benign myositis to rhabdomyolysis can happen quickly, so persistent or worsening symptoms after the first couple of days warrant medical attention. If the pain is improving day over day and the person can gradually bear more weight, that’s the expected trajectory of a benign course.