If a Harmful Variety of Bacteria Is "Allowed" to Establish

If a harmful variety of bacteria is "allowed" to establish a foothold in the body, why does it often then quickly become a full blown infection?

A) The immune system can't easily "see" foreign bacteria.
B) The toxins they produce incapacitate the immune system.
C) They reproduce so quickly the population can get very big very fast.
D) If the bacteria is "allowed" to settle in the first place, the immune system must be weakened already.
E) Our own resident bacteria get recruited into the population once it's settled, and they help spread the infection.