Could there be a pathogen which does not activate an immune response?
&quot;I think, though am not at all sure, that pathogens need to invade cells to be dangerous.&quot;
True in the case of viruses and some bacteria.
Hiding within the cells of the host is the intracellular pathogenesis.
But other &quot;tricks&quot; are possible. Four examples:

biofilms
surface proteins that &quot;catch&quot; antibodies
fast mutation of surface epitopes, like the (in)famous HIV
using host molecules (the HIV again)

Source: Extracellular Immune Avoidance (LibreTexts)
Also interesting: How bacterial pathogens colonize their hosts and invade deeper tissues (Microbes and Infection
Volume 17, Issue 3, March 2015, Pages 173-183).