I may be wrong, but I think that while live rock, corals, and substrates are not hosts, they can be carriers. During the life cycle of ick, ick cysts develop and fall off of the host (fish). These cysts go thru a dormant period until another host is found. If a host is found they can reproduce and infect that new host. If no host is found within a timely manner the cysts will eventually die. I think that the cysts can be carried from one tank to another on live rock, corals, and substrates. That is why the tank must go thru the fallow period. I kind of think of it as little ticking Ick time-bombs with expiration dates.