Ich is far from a flu.
The flu virus does not leave the human body as a part of its lifecycle and then come back into the envirnoment in a much larger wave.
The natural pathogen-host relation of the flu virus depends on incidental contacts, life and death struggle between pathogen and host, and win or lose.
That of ich involves the pathogen naturally limiting itself by leaving the host and then coming back in a large wave, but nearly all die without finding a host. This is why the effect of confinement is far more fundamental for ich.
Ich infestation can be very small so that the human eye cannot detect by observing. Only after a while does it become the size of a period (.), so frequently it cannot be seen. Plus it can be in the gills and so cannot be seen.