Chickenpox Lesion Contagiousness
A patient with a history of chickenpox who has a single pus-filled lesion on their arm is NOT contagious from varicella-zoster virus (VZV), as this represents a secondary bacterial infection complicating a resolved chickenpox infection, not active viral shedding.
Understanding Chickenpox Infectivity Timeline
The key to answering this question lies in understanding when varicella becomes non-infectious:
Patients with chickenpox remain contagious until ALL lesions have completely crusted over 1. This is the critical clinical milestone that determines when isolation precautions can be discontinued.
The typical disease course spans 2-4 weeks from initial symptoms to complete healing, with lesions progressing through predictable stages: macules → papules → vesicles → pustules → crusts 1.
Peak viral shedding occurs in the first 24 hours after lesion onset, with progressive decline as lesions convert to crusts, indicating substantially reduced viral replication by the crusting phase 1.
Why This Single Pus-Filled Lesion is NOT Contagious
Several critical factors indicate this patient is no longer infectious:
The patient has a "history" of chickenpox, meaning the acute infection has passed. If only one lesion remains pustular while the rest have presumably healed, this represents an atypical pattern inconsistent with active varicella 1.
A solitary pus-filled lesion on the arm after chickenpox resolution most likely represents secondary bacterial superinfection (typically Staphylococcus aureus or Group A Streptococcus), not active viral replication 2. Group A beta-hemolytic streptococcus causes 84% of bacterial complications following varicella 2.
True chickenpox presents with multiple lesions in various stages of development (macules, papules, vesicles, crusts) distributed centrally on the face and trunk, then evolving peripherally 1. A single isolated pustule does not fit this pattern.
Critical Distinction: Bacterial vs. Viral Pustules
The presence of pus requires careful interpretation:
Bacterial infections produce thick, whitish-yellow pus and represent pyogenic (pus-forming) infections that are NOT contagious for varicella 3.
Active varicella lesions contain clear vesicular fluid initially, which may become cloudy as they progress to pustules, but this occurs across multiple lesions simultaneously, not in isolation 1.
If this were active varicella, you would expect multiple lesions at different stages, fever, and systemic symptoms—not a single pustule weeks after initial infection 1.
When to Suspect Ongoing Viral Activity
Consider the patient still contagious ONLY if:
Multiple new vesicular lesions continue to appear beyond the typical 4-6 day eruption period 1.
The patient is immunocompromised, as these individuals may develop new lesions for 7-14 days and heal more slowly, with potential for chronic ulcerations and persistent viral replication 4.
Any lesions remain uncrusted—even one active vesicle technically maintains infectivity, though a single outlier pustule weeks after resolution more likely represents bacterial superinfection 1.
Clinical Management Recommendations
For this specific presentation:
Treat the pustule as a bacterial skin infection with appropriate antibiotics targeting Staphylococcus aureus and Streptococcus pyogenes 2.
No isolation precautions are needed if all other chickenpox lesions have crusted and this represents a solitary late complication 1.
Monitor for signs of serious bacterial complications including cellulitis, abscess formation, or necrotizing fasciitis, which can occur as varicella complications 2.
Consider wound culture if the lesion fails to respond to empiric antibiotics, as this confirms bacterial etiology and guides targeted therapy 2.
Common Pitfall to Avoid
Do not confuse a secondary bacterial infection with active viral shedding. The presence of a single pustule weeks after chickenpox onset, in the absence of new vesicular lesions, systemic symptoms, or immunocompromise, represents bacterial superinfection rather than ongoing varicella contagiousness 3, 2.