Small-particle aerosols appear to be particularly important, but other routes are possible. Influenza causes explosive outbreaks of febrile respiratory illness and is an important nosocomial pathogen in hospitals and long-term care facilities. The attack rate in institutionalized adults is as high as 50 percent. The incubation period for influenza virus averages two days. Classic influenza is distinguished by abrupt onset of prominent systemic symptoms, including fever, chills, headache, myalgia, malaise, and anorexia.
Fever usually lasts for an average of three days in adults.
Photophobia and pain with eye movement are frequent early in the illness. Mild conjunctivitis, clear nasal discharge, pharyngeal injection, and small, tender cervical lymph nodes are also common. As systemic illness abates, respiratory symptoms become more apparent. Exacerbations of asthma and other types of preexisting airway disease are often severe. Infections may be subclinical or cause milder illness, including colds.