Whenever injury or illness strikes, runners are concerned about the impact on training. The country is now in the midst of flu season and this is playing havoc with training for spring races.

Influenza is a seasonal viral infection characterized by fever, chills, headache, sore throat, cough, muscle aches and fatigue. Symptoms often develop abruptly and last several days to several weeks. When acutely ill, most runners feel so poorly that they usually don’t try to “run through it”—but when should someone return to training?

You can’t find a great deal of information in the medical literature on this. What we do know: Fever impairs muscle strength, aerobic power and coordination; it also impairs temperature regulation and fluid balance. The immune system is stressed by the illness and cannot handle the stress of strenuous exercise in a normal manner. Increased replication of the virus can occur, worsening the symptoms.

Running when you have a fever may increase the likelihood of developing myocarditis—an inflammation of the heart muscle. There is little data available in humans but this has been demonstrated in mice. Myocarditis goes away by itself in the majority of cases, but due to irritability of the heart muscle, if you develop it you will need to be held out of exercise for at least six months. Serious chronic heart damage persists if myocarditis doesn’t resolve: There is no cure. Bottom line: DO NOT workout when you have a fever!

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Other possible complications of influenza include pneumonia (viral or secondary bacterial infection) and bronchitis with respiratory symptoms lasting a month. Older runners (> 65 years of age) are at greatest risk of serious complications. Asthma, diabetes and heart problems also increase the risk of complications from the flu.

Unless a secondary bacterial infection develops, antibiotics do not play a role in treatment of influenza. You can take acetaminophen or ibuprofen to treat fever and pain. Rest and fluids are recommended. If you test positive for the flu and symptoms have been present for less than 48 hours, an antiviral medication might be prescribed. This can shorten duration of symptoms by a day or two.

When fever, fatigue and muscle pain have resolved, you can resume training. Remember: Deconditioning begins after 4–5 days of inactivity. The following recommendations are a common sense approach because there is little scientific data available:

  • Begin with a short recovery run. Stop if symptoms develop. Repeat this for each day of missed training, increasing the time a little for each successive run.
  • If you don’t feel sick or sluggish during or following these workouts, resume training where you left off. Don’t be tempted to “make up” for time off and jump ahead in a progressive workout plan.
  • If running causes excessive fatigue, keep workouts light until this resolves.

Pushing through symptoms may cause postviral syndrome—persistent fatigue and difficulty training that can last for days to months. Rest is the treatment for this disorder. Read: No Running. Staying on schedule despite illness can have significant consequences.

Prevention is the best “treatment” of influenza. The flu is a tricky virus that has the ability to change its characteristics from season to season, which is why the flu vaccine is not perfect. The vaccine is about 50% effective in preventing the flu this year and can ameliorate symptoms if infection occurs. Avoid contact with infected individuals—and if sick with the flu, stay home! Frequent handwashing can also be helpful.

Be smart when training with an illness. While a cold will have minimal impact on training, more significant illnesses like the flu need to be taken seriously.

Dr. Cathy Fieseler is the director of Sports Medicine in the Trinity Mother Frances Health System in Tyler, TX where she provides medical care for many local high school, college and recreational athletes. She is on the board of directors of the International Institute of Race Medicine (IIRM), and formerly served as chairperson of the Road Runners Club of America Sports Medicine Committee. She also competes in marathons and ultramarathons herself, when she can fit them around her work schedule.