A 6-year-old boy presents with cough, sore throat, and fever. His grandmother, who was recently visiting from Mexico, had similar symptoms. He also has nausea and started having diarrhoea this morning. On examination, he seems ill but is awake and responsive, with a temperature of 38.8°C (101.9°F). His lung examination is clear, and his WBC count is only slightly elevated.
Patients infected with H1N1 swine influenza A in the 2009 pandemic have had variable presentations. The earliest cases in the pandemic occurred in Mexico, where severe pneumonia and high rates of hospital admissions and deaths were reported.  Although severe respiratory failure and fatalities continue to be reported in affected countries, most cases worldwide have been characterised by a relatively mild respiratory illness that resolves with or without medical treatment by 7 to 10 days. The most common symptoms are fever, cough, sore throat, and malaise. Among infants and young children, respiratory symptoms may not be present and the illness may manifest itself as extreme irritability, dehydration, altered mental status, and shortness of breath. Furthermore, neurological complications (including seizures and encephalopathy) have been reported with influenza infections, mainly in children, including during the 2009 H1N1 influenza outbreak.