The ability to perform aerobic metabolism, the primary energy source during long-distance training and racing, is impaired during infection. Studies on rats forced to exercise during an experimental infection show a substantial decrease in performance-related metabolic capacity.
Concentration of enzymes integral to aerobic metabolism are diminished in muscle biopsies of infected humans. Lactate threshold (anaerobic threshold) and maximal oxygen (VO2max) uptake are depressed during and following an infection. It can take one to three months for an athlete to fully recover pre-illness levels of aerobic enzymatic efficiency.
Myocarditis, inflammation of the heart muscle, is one of the more common and most serious complications of an acute (usually febrile) infection. While many viruses and some bacteria can cause myocarditis, a family of viruses called the enteroviruses are the most common culprits.
Unfortunately, a good physical exam and history performed by your doctor is unlikely to determine the exact cause of your fever and muscle aches. Your doctor won't be able to tell you whether your symptoms are caused by a rhinovirus (causes the common cold and is rarely associated with myocarditis) or an enterovirus (which carries a relatively high risk of myocarditis).
Myocarditis without obvious symptoms is relatively common. In a careful epidemiological study of Finnish military recruits performed during the 1980s, the annual incidence of unequivocal myocarditis was a modest 0.02 percent. However, during acute infections, 1.2 percent of sick recruits had evidence of myocarditis on electrocardiograms and blood tests. During an epidemic of influenza A, the incidences rose to 7.7 percent. The majority of affected recruits had no cardiac symptoms.
The relative frequency of myocarditis during the flu wouldn't be a big deal except for an important fact: Vigorous exercise worsens this potentially life-threatening condition. Animal experiments demonstrate that exercise increases viral replication in heart muscle, resulting in more inflammation and destruction of heart muscle.
Sudden death during exercise is often caused by myocarditis. At autopsy, the hearts of athletes who suffered sudden, unexpected deaths often show microscopic signs of myocarditis, including inflammation and dying cells. Genetic material from a variety of viruses and bacteria has been isolated from these inflamed hearts. An arrhythmia (disturbance of heart rhythm) caused by myocarditis is the presumed mechanism of death in these otherwise young, healthy people.
Between 1979 and 1992, 16 elite Swedish orienteers suffered unexpected cardiac death. The majority of these athletes had blood tests consistent with a recent infection and autopsy evidence of myocarditis. Since instituting strict prohibitions against exercising while ill, no further Swedish orienteers have suffered sudden death.
Even after an episode of acute myocarditis is over, there can be long-term consequences. Inflammation can scar the heart muscle. These scars can be where potentially fatal arrhythmias start and these heart rhythm disturbances can change or even end your life. This process of inflammation and subsequent scarring of the heart muscle is one of the hypotheses for the origin of Greg Welch's career-ending difficulties with ventricular tachycardia.
In summary, hard exercise when you have anything more serious than a cold is unlikely to result in significant physiological gains. Training under these circumstances will be of lesser quality, and may even lead to serious consequences.
You are more likely to perform well in your goal race--and in life--if you occasionally learn to kick back and rest, rather than trying to fit in one more hard workout.