Do all pathogens cause disease? If not why? I was under the impression that an infectious disease is caused by a pathogen, but my text book tells me not all pathogens cause disease.
Firstly, a pathogen is a "disease-causing organism or agent". However, this does not necessarily mean that every pathogen brings about disease in a host; rather, they may have the potential to cause disease (pathogenicity), but the host's immune responses prevents it from doing so. Or sometimes, they only cause disease under certain conditions/circumstances (e.g. opportunistic pathogens).
This is because a disease is defined as that which harms the normal functioning of the body. So if an organism is not 'harming' the body, then it's not acting as a "disease-causing agent".
Edit: yes, an infectious disease is always caused by a pathogen, by not all pathogens get the chance to cause disease in a host organism.