Snapshot A 26-year-old woman presents to the emergency room for 3 days of high fever, chills, muscle aches, a severe headache, and sore throat. She works in a nursing home but has not had her flu shot yet this year. She has a past medical history of systemic lupus erythematosus and hypothyroidism. A rapid strep test is negative, and a rapid antigen detection test for flu is positive. She is started on oseltamivir for this infection. Introduction Classification influenza virus an enveloped, segmented, negative-sense, single-stranded RNA virus with a helical capsid an orthomyxovirus transmission via respiratory droplets 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 target of the influenza vaccine neuraminidase allows for progeny virion release from cells Genetics influenza viruses have 8 segments in the genome genetic/antigenic shift reassortment of viral genome segments, such as between human and swine strains, can cause pandemics genetic/antigenic drift random mutations in the H or N gene can cause epidemics 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 Prognosis may lead to severe bacterial superinfections most commonly S. aureus, S. pneumoniae, and H. influenzae may be fatal 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 Making the diagnosis based on clinical presentation and confirmed with laboratory studies 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.) 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