system
Given the following post, respond as if you're commenting on Reddit:
human
How infected does the body have to be in order to trigger an immune response that you notice? I. E. Fever, congestion, cough, etc.
gpt
It's not such a simple equation.  But in general, all it takes is one and your body will start responding to the threat.  The response might be all out war, or it might be to just encapsulate it.  That depends on other things.  Some general categories are:
1.) The type of invader. (Bacterial, Viral, Allergen, etc.)

2.) Where the invader, invades your body.  (E. Coli is fine in some parts and not in other parts.)

3.) How aggressive the invader is. (Take the strain of nora-virus that has hit many of the states really hard the past 12 months.  It has taken as few as breathing in 3 of the virus to infect you.)

4.) The individuals sensitivity to the invader.

5.) The current strength or health of the immune system.