Key Words Pathogen Opportunistic pathogen Adhesion Normal microbiota Penetration Pathogenicity Invasiveness/spread Extra/intra cellular bacteria Virulence Exotoxin Opportunistic infection Nosocomial infection Toxoid Koch's postulates Endotoxin Carrier Transmission Compromised host Outbreak, Epidemic, Pandemic Bacterial biofilmKey Words Adhesion Penetration Invasiveness/spread Extra/intra cellular bacteria Exotoxin Toxoid Endotoxin Carrier Compromised host Bacterial biofilm Pathogen Opportunistic pathogen Normal microbiota Pathogenicity Virulence Opportunistic infection Nosocomial infection Koch’s postulates Transmission Outbreak, Epidemic, Pandemic