"Flu" is a generic term for a respiratory infection produced by several distinct influenza viruses, according to the National Institutes of Health (NIH). More than 200,000 people are hospitalized and about 36,000 people die from the flu in the United States each year. After the acute illness has resolved, flu is often associated with lingering after-effects.
Although exact estimates of prevalence are not available, the NIH cautions that bacterial pneumonia is a frequent after-effect of flu. Airways and lung tissue damaged by the influenza virus provide the perfect breeding ground for a variety of bacterial species. In the most common scenario, a person begins to feel better only to experience an abrupt deterioration a few days later. Symptoms of bacterial pneumonia include fever, chills, cough and difficulty breathing. One easy-to-recognize sign of bacterial pneumonia is green or yellow sputum production, often in copious amounts. The cough associated with influenza is characteristically dry.
Exacerbation of Chronic Disease
Recovery from influenza exacts a toll on the immune system, heart and lungs. Although healthy adults rarely experience any lasting effects, the elderly and people with chronic medical problems are in a different situation. According to "Harrison's Principles of Internal Medicine," these people are most likely to require hospitalization and require adjustment of other kinds of medications. In some cases influenza infection can precipitate cardiovascular, respiratory or kidney failure, which may be irreversible.
According to 2004 article in the journal "Brain," encephalitis lethargica was first described following the pandemic flu of 1918. The syndrome includes symptoms of insomnia, lethargy, uncoordinated movements and involuntary movements, characterized by sudden onset several weeks or months after influenza infection. It was initially believed to reflect a delayed presentation of influenza infection of the central nervous system. However, using more modern techniques, researchers have demonstrated the absence of influenza genetic material in the brains of affected people, suggesting that the true cause of the syndrome may be an autoimmune reaction.