Bacterial vs Viral vs Fungal Infection

2 · Differential / Common Traps

• Assuming bacterial cause from leukocytosis alone — viral illness, steroids, and physiologic stress all raise the WBC.
• Treating green sputum as proof of bacterial bronchitis — purulent sputum reflects neutrophil activity, not organism type.
• Missing fungal disease in an immunocompromised host who is not improving on antibacterials — consider endemic mycoses, Pneumocystis, and invasive mold.
• Attributing persistent fever to antibiotic failure without considering viral co-infection, drug fever, or non-infectious cause.
• Treating colonization as infection — Candida in sputum or urine in a non-immunocompromised, non-critically ill patient is usually colonization, not invasive disease.

3 · Workup and Interpretation

• Syndrome classification first: respiratory, urinary, skin/soft tissue, CNS, bloodstream, GI — this narrows the likely organisms before any test.
• Host factors: immunocompromise (HIV, transplant, steroids, chemotherapy), recent antibiotics, hospitalizations, prosthetic material, travel, animal exposures.
• Respiratory viral panel (influenza, RSV, SARS-CoV-2, others) when viral etiology is plausible — a positive result changes management and may support antibiotic avoidance.
• Fungal testing is targeted, not routine: beta-D-glucan, galactomannan, Cryptococcal antigen, Histoplasma/Blastomyces/Coccidioides urine antigen, or direct culture/biopsy depending on syndrome and host.
• Procalcitonin and CRP are probability modifiers — they do not confirm or exclude bacterial vs. viral etiology on their own.

4 · Treatment / Management

5 · Expected Course / Reassessment

• Bacterial infections typically show clinical improvement within 48–72 hours of appropriate antibiotics — failure to improve should prompt reassessment of diagnosis, source control, and organism susceptibility.
• Viral infections are generally self-limited — most respiratory viral illnesses resolve within 7–14 days. Prolonged or worsening course raises concern for secondary bacterial infection or an alternative diagnosis.
• Fungal infections often have a more indolent course and may require weeks to months of antifungal therapy. Response is assessed by clinical improvement, biomarker trends, and repeat imaging or cultures.

6 · Escalation

• Failure to improve on appropriate antibacterials in an immunocompromised host — broaden the differential to include fungal and atypical organisms.
• Suspected invasive fungal disease — ID consultation, targeted fungal testing, and antifungal initiation.
• Severe viral illness (influenza with respiratory failure, disseminated HSV, CMV disease in transplant) — antiviral therapy and specialist involvement.
• Any patient with hemodynamic instability, worsening organ function, or failure to respond to initial therapy — reassess the diagnosis, source, and organism before escalating empirically.

Apply It · Patient Cases

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