The average adult has two to three upper respiratory infections each year. Many athletes wonder if they should continue their training routine when sick. While research is limited, most experts recommend that if your symptoms are above the neck and you have no fever, exercise is probably safe. Intensive exercise should be postponed until a few days after the symptoms have gone away. However, if there are symptoms or signs of the flu, such as fever, extreme tiredness, muscle aches, swollen lymph glands, then at least two weeks should probably be allowed before you resume intensive training.