Influenza

Rhinovirus, parainfluenza, adenovirus, human metapneumovirus, and the endemic coronaviruses cause the great majority of upper and lower respiratory tract infections outside influenza and RSV, producing overlapping syndromes (common cold, croup, tracheobronchitis, pharyngoconjunctival fever, pneumonia) that are diagnosed clinically and managed supportively in most patients. Testing and antiviral therapy are reserved for hospitalized, immunocompromised, or diagnostically uncertain cases.

Community-acquired pneumonia is a lower respiratory tract infection acquired outside the hospital that is diagnosed by a compatible clinical syndrome plus a new infiltrate on chest imaging, risk-stratified with CURB-65 or the Pneumonia Severity Index to guide site of care, and treated with empiric antibiotics chosen by site of care and comorbidity, escalating to cover MRSA or Pseudomonas aeruginosa when risk factors are present.

Acute bronchitis is a self-limited lower respiratory tract infection causing cough lasting more than five days after other upper respiratory symptoms resolve, almost always viral, and diagnosed clinically after pneumonia is excluded. Management is symptomatic; antibiotics do not shorten the illness and should be withheld absent signs of pneumonia or pertussis.

COVID-19 is the respiratory illness caused by SARS-CoV-2, a positive-sense RNA coronavirus that binds ACE2 receptors, ranging from asymptomatic infection to hypoxemic pneumonia and acute respiratory distress syndrome. Diagnosis relies on RT-PCR of a nasopharyngeal sample, and hospitalized hypoxemic patients are treated with dexamethasone plus remdesivir, escalating to baricitinib or tocilizumab and thromboprophylaxis.

Respiratory syncytial virus is the leading cause of bronchiolitis and infant hospitalization for lower respiratory tract infection, peaking in fall and winter in children under two years. Diagnosis is clinical; management is supportive, and prevention now centers on universal infant immunoprophylaxis with nirsevimab or maternal RSV vaccination.

Whooping cough is a highly infectious bronchitis caused by the gram-negative coccobacillus Bordetella pertussis, marked by catarrhal, paroxysmal, and convalescent stages with lymphocytosis; diagnosis is clinical, confirmed by nasopharyngeal PCR, and treated with azithromycin plus prophylaxis of close contacts.

Sepsis is life-threatening organ dysfunction caused by a dysregulated host response to infection, identified by an acute rise in SOFA score of 2 points or more in a patient with known or suspected infection. Survival depends on recognizing the pattern early and completing the hour-1 bundle: lactate, blood cultures before antibiotics, broad-spectrum antibiotics such as cefepime 2 g IV every 8 hours, and 30 mL/kg crystalloid for hypotension or a lactate of 4 mmol/L or higher.

Myocarditis is inflammation of the myocardium, most often postviral or immune-mediated, presenting across a spectrum from a mild chest-pain syndrome mimicking myocardial infarction to fulminant heart failure and arrhythmia. Diagnosis relies on cardiac MRI (or biopsy for severe or refractory cases), and treatment is supportive heart-failure care, with cause-specific immunosuppression for giant cell, checkpoint-inhibitor, and autoimmune myocarditis.

ARDS is acute (within 1 week), bilateral, non-cardiogenic hypoxemic respiratory failure from diffuse alveolar damage, diagnosed by the 2012 Berlin criteria and graded by the Pao2/Fio2 ratio. Management is lung-protective ventilation, PEEP titration, early prone positioning for severe disease, conservative fluid strategy, and treatment of the inciting insult.