If a researcher introduced a suspected pathogen into many healthy hosts, but none of them became sick, what could this indicate?

This would demonstrate this isn't the right pathogen that has been causing illness. It could imply that the hosts have a characteristic insusceptibility or the pathogen is a symbiont, or not harmful microscopic organisms.

The suspected pathogen must be found for each situation of ailment and not be found in healthy people. It can be isolated and developed in pure culture.