MODES AND MECHANISMS OF TRANSMISSION OF PATHOGENIC ORGANISMS IN THE HEALTHCARE SETTING AND STRATEGIES FOR PREVENTION AND CONTROL. Chain of Infection Infectious agent or Pathogen: A biological, physical, or chemical agent capable of causing disease.  Biological agents may be Bacteria; e.g. Staph, Strep, MRSA, E. coli, etc.Viruses; e.g. Hepatitis B & C, HIV, flu, novel...