What, other than genetic factors, influence which individual survives a parasite and which perishes?
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).