Presumably non-susceptible hosts do not supply the right stimuli. Host immune response. Parasites have developed a number of methods of avoiding their hosts' immune response. Some such as schistosomes incorporate host antigen into their teguments and so are not recognised by their hosts as foreign. Others like the African Trypanosomes keep changing their antigenic surface and so stay one jump ahead of the hosts' immune response.
Many potential hosts show innate immunity to infection.
Innate immunity is an active response on the part of the host, but unlike acquired immunity it is not dependent on previous infection. The actual cellular and humoral mechanisms involved in innate resistance are not well understood, but include serum factors such as high density lipoprotein and complement as well as natural killer cells and platelets. It seems likely that parasites are restricted more by their overall nutritional requirements than by the need for obscure or specific metabolites. The molar ratios of amino acids in the vertebrate intestine are remarkably constant and this is related to the amino acid uptake mechanisms in the mucosa. Parasites such as tapeworms or acanthocephalans that rely on their hosts digestive processes to provide them with low molecular weight nutrients have to compete with the uptake mechanisms of their host's intestine.