Influenza virus may be one of three basic types (A, B, or C), depending on the number of gene segments surrounding the RNA viral core. H1N1, commonly referred to as swine flu virus, is one of the most common prevailing subtypes of influenza A, a zoonotic infection with over 100 types infecting most species of birds, pigs, horses, dogs, and seals. In humans, it is spread by respiratory secretions from an infected person. The virus then invades and replicates in the airway and respiratory tract cells, causing systemic symptoms from inflammatory mediators. The incubation period of influenza ranges from 18 to 72 hours.

Complications from influenza H1N1 include acute respiratory distress, conjunctivitis, myocarditis, pericarditis, pneumonia, sepsis, organ failure, and death.

Patients infected with influenza H1N1 may present with mild symptoms or severe illness. Initial symptoms may include high fever (as high as 104° F [40° C]), mild to severe myalgia, rhinorrhea, and sore throat. Tachycardia may occur from hypoxia, fever, or both. The skin may be warm or hot, depending on temperature status. Nausea and vomiting may occur, and diarrhea is common in children. This may produce signs of dehydration. Frontal orbital headache is often severe and accompanied by photophobia, burning sensations, or pain upon motion. Weakness and severe fatigue may prevent patients from performing their activities of daily living and lead to the need for bed rest. A nonproductive cough, pleuritic chest pain, and dyspnea may occur. Acute encephalopathy may occur in patients with influenza A virus, producing altered mental status, coma, seizures, and ataxia.