What is the difference between bacterial and viral infections?

Differentiating Bacterial from Viral Infections

Bacterial infections persist beyond 10 days without improvement, present with severe onset (fever ≥39°C with purulent discharge for 3+ consecutive days), or demonstrate "double-sickening" (worsening after initial improvement), while viral infections typically peak at 3-6 days and resolve within 7-10 days. 1, 2

Clinical Trajectory: The Primary Discriminator

Duration and pattern of symptoms are more reliable than any single laboratory test or clinical feature. 1, 2

• Viral infections typically improve gradually after peaking at days 3-6, with complete resolution within 7-10 days 3, 2
• Bacterial infections follow one of three patterns:
  • Symptoms persist ≥10 days without clinical improvement 3, 1
  • Severe onset with high fever (≥39°C) AND purulent discharge or facial pain for ≥3-4 consecutive days at illness beginning 3, 2
  • "Double-sickening": new fever, headache, or increased discharge after initial improvement at days 5-7 3, 1, 2

Critical Pitfalls to Avoid

Purulent discharge color does NOT indicate bacterial infection—both viral and bacterial infections produce purulent secretions due to neutrophil influx, not bacterial presence. 3, 2 Nasal purulence alone is a sign of inflammation and is not specific for infection. 3

Fever intensity does NOT distinguish bacterial from viral infection—both can present with high fevers. 1, 2 However, fever persisting >3-4 days increases the likelihood of bacterial etiology. 1, 2

Associated upper respiratory symptoms (rhinorrhea, nasal congestion, sneezing) favor viral etiology, as viral rhinosinusitis is characterized by these features. 3, 2

Laboratory Differentiation: Limited Utility

Traditional acute phase reactants (CRP, WBC) have wide overlap between bacterial and viral infections and should not be used as sole discriminators. 1, 4

• Procalcitonin <0.25 ng/mL has high negative predictive value for ruling out bacterial infection 4
• Procalcitonin >0.5 ng/mL with neutrophil predominance strongly suggests bacterial infection 4
• Serial procalcitonin measurements are more valuable than single measurements 4
• Multiplex PCR for respiratory pathogens reduces antibiotic use by 22-32% when viral pathogen detected 3, 4

Site-Specific Differentiation

Meningitis

• CSF neutrophilic predominance with CSF:plasma glucose ratio <0.5 indicates bacterial meningitis 3, 1, 4
• CSF lymphocytic predominance with normal glucose strongly suggests viral meningitis 3, 1, 4
• CSF lactate <2 mmol/L effectively rules out bacterial meningitis 1

Respiratory Tract

• Simple chronic bronchitis exacerbation without obstructive disease does not require immediate antibiotics even with fever 1
• Presence of ≥2 of 3 Anthonisen criteria (increased dyspnea, sputum volume, sputum purulence) suggests bacterial infection 1
• Hypoxemia (SpO2 <92%) in pneumonia increases risk of bacterial etiology and mortality 1

Treatment Algorithm

For Suspected Viral Infection (symptoms <10 days, gradual improvement)

• Do NOT prescribe antibiotics—they provide no benefit for viral illness and do not provide direct symptom relief 3, 1, 5
• Provide symptomatic relief: analgesics (acetaminophen, ibuprofen), topical intranasal steroids, nasal saline irrigation 3, 2
• Oral decongestants may provide relief (avoid in hypertension/anxiety); topical decongestants should not exceed 3-5 days to avoid rebound congestion 3

For Suspected Bacterial Infection (meeting criteria above)

• Initiate antibiotics when bacterial criteria are met 3, 1
• First-line for respiratory bacterial infections: amoxicillin or amoxicillin-clavulanate for 5-7 days 3, 1, 5
• Doxycycline or respiratory fluoroquinolone as alternatives 3

For Uncertain Cases

• Observe for 2-3 days before initiating antibiotics 1, 2
• Reassess if fever persists >3 days or symptoms worsen 1, 2
• Provide symptomatic management during observation period 3, 2

For Bacterial Meningitis (requires immediate action)

• Initiate antibiotics within 1 hour, before imaging or lumbar puncture 1
• Adults <50 years: ceftriaxone or cefotaxime 1
• Adults ≥50 years or Listeria risk factors: add amoxicillin to cephalosporin 1
• Neonates: amoxicillin plus cefotaxime 1

Key Pathophysiologic Differences

Viral infections promote vigorous inflammatory response causing epithelial disruption, edema, and excessive mucus production that impairs ciliary function—this explains the purulent discharge without bacterial presence. 3 Secondary bacterial infection complicates only 0.5-2.0% of viral upper respiratory infections. 3

Bacterial sinusitis results from obstruction of sinus ostia following viral infection, leading to impaired mucosal clearance. 3 The most common bacteria are S. pneumoniae and H. influenzae, accounting for ~70% of cases. 3