How come some pathogenic micoorganisms are commensals in our body?

[sameeralord]

For example candida fungus is a commensal in our skin and mucus membrane, but it can cause infection. So I'm asking why is it not causing infection in everyone, are the commensal form of candida harmless, so to get the infection we must inhale patogenic candida. Thanks

 

[nmz787]

depends where it is... strep is on our skin all the time, but if i lick my finger, then choke a bit and inhale some saliva, now its in my lungs which is a totally different environment where it may be easier for the bacteria to infiltrate the blood system and spread infection which is detected by swelling of the lymph nodes.

This has some hover-over stuff, looks pretty decent to me:
roarproject.org/ROAR/html/commensal.htm