Every year in Winter we can observe influenza burst out and many people who struggle with new flu viruses and infections. Cold winds, humidity and lack of UV affecting human immune system in a bad way, moreover very often we are prone ton catch viruses and bacteria because of contact with other people by using public transport or at work, at school, in swimming pools, leisure facilities and gyms.
I think that once, everyone ignored the symptoms and then suffered because of their poor choice... Only in last 8 weeks I saw many people doing training when ill and this time I have decided to explain why training during infection is a waste of time.
Acute infection results in production of adrenaline and cortisol and elevated levels of histamine, bradykinin, and prostaglandins. During infection body becomes catabolic as all nutrients are mobilised (e.g., amino acids) and the body breaks down protein from muscle, which are scavenged by liver and used as an emergency energy source and to be used as acute phase proteins to fight infection. The same chemicals that initiate muscular break down can inhibit repair and protein absorption making the tissue impossible to recover over the time of inflammation.
Your muscles have many good reasons to ache when you have an infection. Skeletal muscle as the main source of catabolised protein including heart muscle contributing as well. Ignoring symptoms that could be attributable to the heart, including shortness of breath, irregular heartbeat, fainting, chest pain or lightheadedness at the time of illness can be really bad idea. Myocarditis, inflammation of the heart muscle, is one of the more common and most serious complications of an acute (usually febrile) infection.
Fat metabolism is impaired during infections, what causes the sick person to rely more on muscle as an energy source, than it would during times of physical stress (e.g., starvation or heavy training) in normal conditions. Weight loss during a febrile illness is unfortunately lean muscle mass.
In summary, exercise when you have anything more serious than a cold is unlikely to result in significant physiological gains due to human physiology. Forced training when ill will be of lesser quality and may even lead to serious consequences.
So instead of adding additional scoop, drink one more coffee or God knows what to enhance your performance during the illness, rather invest in hot drink, paracetamol and proper rest.