What factors other than "good genes" are at play in determining whether one individual survives a parasite and another individual in the same species perishes as a result of the same parasite? For example, is there variation in the amount of the parasite (not in the amount of parasite species but in the amount of one species) with which an individual is infected and how might this occur? I'm mainly interested in this first example, but another example: Might infection occur at different parts of life-history in which vulnerability varies? (I am asking generally about any parasite species and any host species.) Thank you very much.
I am neither a parasitologist nor an immunologist, so these suggestions are only provided as a starting point and may meet valid criticisms from others. However, as your comment mentions the 1918 flu epidemic, a couple of general suggestions.
Assuming that resistance to pathogenic organisms can involve the immune response, then at least two factors that might affect that — and hence resulting mortality or morbidity are:
Nutrition: It is popular wisdom that undernourished individuals are more susceptible to disease. This might have diverse causes, but there would appear to be an extensive literature on the role of nutrition in the immune response, e.g. this article by Chandra has been highly cited.
Previous immune challenges: There is, of course, the classic case of milk maids being resistant to smallpox because of immunity provided by the related cowpox. Thus exposure to previous strains of a virus may provide some protection. Speculating widely here, there may also be affects on the non-specific components of the immune response (macrophages and whatever).