Psittacosis

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.

Mycoplasma pneumoniae is a cell-wall-deficient atypical bacterium and a classic cause of walking pneumonia in young, previously healthy people in close-contact settings, notable for a gradual onset, cough out of proportion to exam and imaging findings, beta-lactam resistance, and a distinctive set of extrapulmonary manifestations.

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.

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.

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.

Tularemia is a zoonosis caused by the gram-negative coccobacillus Francisella tularensis, acquired from rabbits, ticks, deer flies, or inhaled aerosols, that presents with abrupt fever and either an ulcer with tender regional lymphadenopathy or pneumonia with hilar adenopathy and pleural effusion. Diagnosis rests on exposure history confirmed by serology or PCR rather than culture, which endangers laboratory staff; severe disease is treated with streptomycin or gentamicin and nonsevere disease with doxycycline or ciprofloxacin.

Influenza is an acute orthomyxoviral respiratory illness with abrupt fever, myalgia, and cough, driven by an H and N surface glycoprotein antigenicity that drifts yearly (seasonal epidemics) and occasionally shifts (pandemics). Diagnosis is clinical or confirmed by nucleic acid amplification testing, and neuraminidase-inhibitor or endonuclease-inhibitor antiviral therapy started within 48 hours of onset shortens illness and reduces complications in high-risk patients.

Chlamydia trachomatis is the most commonly reported bacterial sexually transmitted infection in the United States, frequently asymptomatic, and diagnosed by nucleic acid amplification testing of urine or genital/rectal/oropharyngeal swabs. Uncomplicated genital infection is treated with a single dose of azithromycin or a course of doxycycline, complications include pelvic inflammatory disease and epididymitis, and LGV serovars require a 21-day course and are an increasing cause of proctitis.