All encounters with pathogens necessarily lead to a state of disease.
The immune system mounts a defense to counter the pathogen and manages to control the infection, in the first instance. In addition to suppressing the pathogen for the first time, our immune system also learns something about the pathogen that can be used against it, in the second round of infection. So the immune system is the reason why all encounters with pathogens do not necessarily lead to a state of disease.