Influenza results from three types of myxoviruses. Type A, the most prevalent, strikes every year, with new serotypes causing epidemics every 3 years. Type B also strikes annually but only causes epidemics every 4 to 6 years. Type C is endemic and causes only sporadic cases.

The infection is transmitted by inhaling a respiratory droplet from an infected person or by indirect contact such as drinking from a contaminated glass. The virus then invades the epithelium of the respiratory tract, causing inflammation and desquamation.

One remarkable feature of the influenza virus is its capacity for antigenic variation —that is, its ability to mutate into different strains so that no immunologic resistance is present in those at risk. Antigenic variation is characterized as antigenic drift (minor changes that occur yearly or every few years) and antigenic shift (major changes that lead to pandemics).

The most common complication of influenza is pneumonia, which can be either primary influenza viral pneumonia or pneumonia secondary to bacterial infection. Influenza also may cause myositis, exacerbation of chronic obstructive pulmonary disease (COPD), Reye’s syndrome and, rarely, myocarditis, pericarditis, transverse myelitis, and encephalitis. High-risk individuals are those older than age 65 with preexisting COPD, coronary artery disease, diabetes mellitus, chronic metabolic disorders, or chronic renal failure.

The patient’s history usually reveals recent exposure to a person with influenza. Most patients say that they didn’t receive the influenza vaccine during the past season.

After an incubation period of 24 to 48 hours, flu symptoms appear. The patient may report sudden onset of chills, fever (101° to 104° F [38.3° to 40° C]), headache, malaise, myalgia (particularly in the back and limbs), photophobia, a nonproductive cough and, occasionally, laryngitis, hoarseness, rhinitis, and rhinorrhea. These signs usually subside in 3 to 5 days, but cough and weakness may persist. Some patients (especially elderly people) may feel tired and listless for several weeks.

Inspection initially may reveal clear nasal discharge, erythema of the nose and throat without exudate, and red, watery eyes.

As the disease progresses, respiratory findings become more apparent. The patient frequently coughs and looks tired. If pulmonary complications occur, tachypnea, cyanosis, and shortness of breath may be observed. Auscultation may disclose transient crackles. If the patient has pneumonia, breath sounds may be diminished in areas of consolidation. With bacterial pneumonia, you’ll see purulent or bloody sputum.