A common guide is the neck check. If your symptoms are mainly above the neck, such as a runny nose, mild sore throat, or sneezing, gentle to moderate exercise is generally safe and unlikely to cause harm. Listen to your body and ease off if you feel worse.
You should rest and avoid strenuous exercise if you have symptoms below the neck or affecting the whole body, such as fever, muscle aches, a chest cough, breathlessness, vomiting, or significant fatigue. Exercising with a fever or flu can prolong illness and, rarely, strain the heart.
When returning after illness, build back gradually rather than jumping straight to full intensity. If you have an underlying heart or lung condition, check with a doctor before exercising while unwell.