Snapshot A 50-year-old man presents to the emergency room for altered mental status, high fevers, chills, muscles aches, and headaches for 1 week. He was diagnosed with flu at an urgent care center but declined oseltamivir at the time. He had been improving until 2 days ago, when he started having fevers and chills again along with a productive cough. On physical exam, his oxygen saturation is at 90% on room air and is put on supplemental oxygen. Infectious workup, including blood cultures, are pending. A chest radiograph shows a right lower lung consolidation, and he is started on empiric antibiotics. Introduction Classification influenza virus an enveloped, segmented, negative-sense, single-stranded RNA virus with a helical capsid an orthomyxovirus transmission via respiratory droplets Prevention annual flu vaccine for those 6 months or older contains multiple killed viral strains that are thought to be likely to appear during flu season intramuscular live-attenuated vaccine intranasal Epidemiology Incidence annual epidemics, often occurring in the winter Demographics affects both adults and children, though with higher frequency in children Risk factors advanced or young age pregnancy immunosuppression morbid obesity pulmonary disease Etiology Influenza A and B are causes of seasonal epidemics Can originate from animal strains avian flu A swine flu A (H1N1) Pathogenesis surface protein hemagglutinin (H) and neuraminidase (N) hemagglutinin binds to sialic acid and allows for viral entry into cells neuraminidase allows for progeny virion release from cells Presentation Symptoms systemic symptoms predominate high fevers and chills myalgias and malaise headaches nausea and vomiting may have cough or sore throat Physical exam may have cervical lymphadenopathy Imaging Chest radiography indication if pneumonia is suspected findings consolidation or infiltrate ground-glass opacities Studies Labs rapid antigen detection tests (RADTS) most commonly used detection of RNA on reverse transcriptase polymerase chain reaction Differential Upper respiratory infection distinguishing factor upper respiratory symptoms (e.g., cough, congestion, etc.) often predominates rather than systemic symptoms (e.g., high fevers, chills, malaise, etc.) DIAGNOSIS Making the diagnosis based on clinical presentation and confirmed with laboratory studies Treatment Management approach mainstay of treatment is supportive care avoid aspirin in children can cause Reye disease, leading to hepatorenal failure Conservative supportive care indication all patients modalities hydration antipyretics Medical oseltamivir/zanamivir indication patients with severe disease or at high risk of severe disease Complications Secondary bacterial infection Myositis or myocarditis Central nervous system involvement Prognosis May lead to severe bacterial superinfections most commonly S. aureus, S. pneumoniae, and H. influenzae May be fatal