Hi Rabbitears. From my study of this problem of memory immunity [to parasitic worms mostly] as it applies to animal studies [mainly sheep] re-infection occurs more frequently with the younger stock than the older. The antibodies are very antigen specific, so re-infection can occur where antigenic specificity is slightly altered. It takes several infections of the same pathogen\ parasite for the hyper-response mechanisms to sensitize[sp?] Shroom