Primary lesion

Primary effect of syphilis on the underside of the penis

In medicine, a primary lesion or primary effect is a local reaction at the beginning of an infectious disease .

Examples are the primary effect of tuberculosis as a reaction to the pathogen Mycobacterium tuberculosis or the ulcus durum (hard chancre) at the entry point of Treponema pallidum in syphilis .

The primary effect and reacting regional lymph nodes are collectively referred to as the primary complex .