Pneumonia

Legionella pneumophila is an intracellular, water-borne gram-negative bacillus causing severe community-acquired pneumonia with prominent gastrointestinal and hepatic features, confirmed by urinary antigen testing and treated with a macrolide or fluoroquinolone rather than a beta-lactam.

Haemophilus influenzae is a gram-negative coccobacillus whose encapsulated type b (Hib) strain historically caused invasive childhood disease (epiglottitis, meningitis, bacteremia) before routine vaccination, while nontypeable strains now dominate as a cause of mucosal infections (otitis media, sinusitis, COPD exacerbation, pneumonia). Epiglottitis is a clinical airway emergency treated by securing the airway before diagnostic workup, and invasive disease is treated with a third-generation cephalosporin.

Pseudomonas aeruginosa is a nosocomial, opportunistic gram-negative bacillus that causes hot-tub folliculitis, malignant otitis externa, ventilator-associated pneumonia, febrile-neutropenia bacteremia with ecthyma gangrenosum, and device-associated infections. Empiric therapy requires an antipseudomonal agent and, in resistance-risk or unstable patients, double coverage until susceptibilities return.

Coccidioidomycosis, or valley fever, is a dimorphic fungal infection acquired by inhaling Coccidioides arthroconidia in the southwestern United States, ranging from self-limited pneumonia to disseminated or meningeal disease, diagnosed by culture, histopathology, or serology and treated with fluconazole for symptomatic disease and lifelong fluconazole for meningitis.

Histoplasma capsulatum is an endemic dimorphic fungus, acquired by inhalation of conidia in the Mississippi and Ohio River valleys and other endemic regions, that most often causes asymptomatic or mild pulmonary infection but can disseminate in immunosuppressed hosts; diagnosis rests on the Histoplasma urinary antigen assay and treatment is itraconazole for mild to moderate disease or liposomal amphotericin B followed by itraconazole for severe or disseminated infection.

Blastomycosis is a dimorphic fungal infection acquired by inhaling Blastomyces dermatitidis conidia in the Mississippi and Ohio River valleys, presenting as primary pulmonary infection that can mimic lung cancer or disseminate to skin and bone, diagnosed by direct fungal stain confirmed with culture, antigen, or serology, and treated with itraconazole for mild to moderate disease or liposomal amphotericin B for severe, disseminated, or CNS disease.

Invasive aspergillosis is an angioinvasive mold infection of neutropenic and immunosuppressed hosts that begins in the respiratory tract and disseminates hematogenously, diagnosed with CT, galactomannan, and biopsy, and treated first-line with voriconazole.

Tuberculosis, caused by Mycobacterium tuberculosis, remains a leading global cause of infectious death and presents as latent infection, primary/secondary pulmonary disease, or extrapulmonary and disseminated (miliary) disease. Diagnosis rests on AFB smear, NAAT, and mycobacterial culture (the gold standard), and drug-susceptible active disease is treated with a four-drug regimen (isoniazid, rifampin, pyrazinamide, ethambutol) for 2 months followed by isoniazid plus rifampin for 4 to 7 months.

Bacteremia is the presence of viable bacteria in the blood, confirmed by positive blood cultures, that ranges from transient and clinically silent to the trigger for sepsis and septic shock requiring urgent source identification and antimicrobial therapy.

Q fever, caused by inhalation of Coxiella burnetii aerosols from infected livestock, most often produces a self-limited febrile illness with atypical pneumonia or hepatitis, but it is a major cause of culture-negative endocarditis when infection becomes chronic. Acute disease is treated with doxycycline for 2 weeks; endocarditis requires combination therapy for more than 18 months.