Influenza Infection

Influenza A and B are respiratory viruses. Their hemagglutinin attaches to receptors on respiratory epithelial cells, allowing the virus to enter and replicate. Infected cells are injured, while the immune response releases inflammatory mediators that produce fever, chills, headache, myalgia, malaise, and the familiar dry cough. The patient often feels systemically ill because influenza is not confined to the nose and throat; the inflammatory response affects the whole body. Antigenic drift gradually changes viral surface proteins, so immunity from a previous infection or vaccine may not fully recognize the next season’s strains. Antigenic shift is a less frequent, more substantial change associated with influenza A and can create a strain to which population immunity is limited. Most uncomplicated infections resolve without hospitalization. The danger is the change in physiology that can follow: inflammation and epithelial injury may worsen gas exchange, dehydration can reduce circulating volume, and the infection can trigger an exacerbation of asthma, chronic obstructive pulmonary disease, heart failure, or another chronic illness. Secondary bacterial pneumonia is another important complication. An older adult or immunocompromised patient may deteriorate without...