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. 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.