There is ample research. That's what marine biologists do.
Ich can reside in the gills. Mostly it resides in a sandbed. It takes about 12 weeks without access to fish to starve it out of a tank.
A stressed fish will manifest stress often by reduction of the slime coat. Fish with extremely thick slime coats, like mandarins, are nearly immune to ich. Fish with very little slime coat, like tangs and angels, are particularly susceptible to ich. Having bad water quality and particularly low alkalinity is likely to reduce the slime coat of all fish, and make implantation of an ich swimmer-phase far more likely than if the water was good and balanced.
Ich is a multicelled animal, not a virus and not a germ. It has several life stages, one on the fish, inside a pimple, one traveling to the sandbed, living in the sandbed, and traveling up to find a new host. The better the fish's slimecoat, the less likely it will succeed, and if it does not succeed, it dies. Period.