Length of weakness from flu: How long after running no fever will it take till my muscle/joint weakness starts to disappear? When can I resume my normal outdoor activities (cycling) without fear of relapse?
Ann Edmundson, MD, PhD
An otherwise healthy person who gets the flu will usually improve over two to five days. In some people that tired feeling may last for one week or more and very rarely the symptoms can last for several weeks (post influenza asthenia).
A major concern when you get the flu is that complications can arise. This is not strictly a relapse, but the development of another illness. The risk is highest for those with diabetes; heart, lung, or kidney disease; and other chronic diseases that lower the body's resistance to infection. Persons over age 65 are also at greater risk of developing complications from the flu.
Talk with your doctor if your symptoms are not improving. It is always better to wait until you are feeling better before starting back on a vigorous exercise program. It is normal to feel weak for several days after the flu, but if your symptoms persist seek medical attention to be safe.