A secondary infection is a new infection that arises during or after treatment of a primary infection, often because normal host defences are weakened or the microbiota is disrupted.
Explanation
Secondary infections differ from coinfections in that they follow an initial illness rather than occurring simultaneously. The primary infection, its treatment or the host response can damage tissues, suppress immune function or disturb microbial communities, creating niches for opportunistic pathogens. Antibiotics that eradicate susceptible bacteria may allow resistant species or fungi to overgrow. Viral infections such as influenza or varicella impair mucosal barriers and ciliary clearance, predisposing patients to bacterial pneumonia or skin infections. Immunocompromised individuals, including those with HIV, diabetes, burns or undergoing chemotherapy, are particularly prone to secondary infections. Distinguishing a secondary infection from relapse of the original pathogen requires microbiologic testing. Appropriate management involves targeted antimicrobial therapy, restoration of host defences and, when possible, avoidance of unnecessary broad‑spectrum antibiotics. Preventing secondary infections is a major rationale for vaccination, infection control measures in hospitals and judicious antimicrobial use.
Illustrative cases
Bacterial pneumonia due to Staphylococcus aureus or Streptococcus pneumoniae is a well‑known complication of influenza, contributing to high mortality during pandemics. After varicella (chickenpox), scratching and vesicle rupture can lead to secondary infections by Streptococcus pyogenes or Staphylococcus aureus. Broad‑spectrum antibiotic therapy may suppress gut flora and allow overgrowth of Clostridioides difficile, causing colitis. Immunocompromised patients with HIV may develop secondary infections such as oral candidiasis or cytomegalovirus retinitis. Ventilator‑associated pneumonia in hospitalized patients often occurs after initial respiratory illness and is caused by opportunistic Gram‑negative bacteria like Pseudomonas aeruginosa or Acinetobacter species.
Secondary infections underscore the delicate balance between host, pathogen and microbiota. Preventive measures, supportive care and precise antimicrobial therapy are essential to minimize these complications.
Related Terms: Primary infection, Superinfection, Opportunistic pathogen, Immunosuppression, Nosocomial infection