A quote from page 3 of Mandell's "Principles and Practice of Infectious Diseases" (Sixth edition): "It is useful to distinguish "principal" pathogens, which regularly cause disease in some proportion of susceptible individuals with apparently intact defense systems, from other potentially pathogenic microorganisms. ... even for most organisms classified as principal pathogens, for example, Staphylococcus aureus and the pneumococcus, some impairment or local breakdown in normal host defense mechanisms must occur for these bacteria to cause disease. ... Thus, it seems clear that the capacity of certain microorganisms to cause disease in seemingly uncompromised human hosts on a regular basis reflects some fundamental difference in their virulence capabilities from those of opportunists and the more numerous commensal species that rarely, if ever, cause disease."

A pathogen with a primary infectious disposition can cause disease or death in both immunocompromised and immunocompetent hosts.

An infectious disposition to become part of a disorder in organisms that have intact defenses.

Albert Goldfain

Lindsay Cowell

Alexander Diehl