Pathogen
One-Sentence Definition
A pathogen is a microorganism (or infectious agent) capable of causing disease in a host.
Simple Explanation
A microbe that can make you sick — not just live quietly on or in you.
Detailed Scientific Explanation
- Frank (primary) pathogens: cause disease in healthy hosts (e.g., M. tuberculosis, influenza).
- Opportunistic pathogens: cause disease mainly when barriers/immunity fail (e.g., Candida, P. aeruginosa in compromised hosts).
- Pathogenicity = ability to cause disease; virulence = degree of damage / severity.
- Depends on host factors: dose, route, immune status, microbiome context.
- Contrast with Normal Microbiota (usually commensal/mutualist; can become opportunistic).
Mechanism
Virulence factors: adhesion, invasion, toxins, immune evasion (Capsule, antigenic variation), nutrient acquisition. Outcome = pathogen strategies × host response (MOC - Immunology).
Clinical Importance
- Frames every infectious disease differential.
- Guides isolation precautions and empiric therapy by likely pathogen class.
Research Importance
- Molecular Koch’s postulates test candidate virulence genes (Koch’s Postulates).
Diagnostic Relevance
- Goal of lab workup: detect/identify pathogens (or rule them out) in clinical context — not every detected organism is the cause (PCR).
AMR Relevance
- Pathogens under drug pressure evolve resistance; colonizers can be reservoirs of resistance genes.
Related Book Chapters
Related MOCs
Active Recall Questions
- Primary vs opportunistic pathogen?
- Pathogenicity vs virulence?
- Can normal flora act as pathogens? Example?