Once the fever and ill feeling have abated, symptoms of weakness or lethargy may persist for another week or so. The symptoms of a cold are usually completely gone within a week after it began. Most patients with the flu recover uneventfully. However, complications can occur. Even without complications most flu sufferers are very ill and feel fortunate to recover in a week or so.
Influenza is a systemic disease and for a small number of patients major complications can occur.
Bacterial pneumonia was probably responsible for most of the deaths in the great influenza pandemic of 1918. People most prone to develop complications during flu epidemics are the elderly and debilitated, those with chronic diseases, or those have a compromised immune system. Military recruits and others living in a very close quarters are also at increased risk of flu complications. It appears so. And to get it more severely.