The causes and consequences of aggregation among conspecifics have received much attention. For infecting macroparasites, causes include variation among hosts in susceptibility, and/or whether infective stages are aggregated in the environment. Here, we link these two phenomena and explore whether aggregation of infective stages in the environment is adaptive to parasites encountering host condition-linked defenses, and what effect such aggregations have for parasite-host interactions. Using simulation models, we show that parasite fitness is increased by aggregates attacking a host, particularly when investment into defenses is high. The fitness benefit of aggregation remains despite inclusion of factors that should curb the benefits of aggregation: namely, mortality of low condition hosts (those hosts expected to be most susceptible to parasitism) and costs of high coinfection. Using sample sizes common in studies, aggregation of infective stages reduces the likelihood of detecting host condition-parasitism relations, even when host condition is the only other factor in models affecting parasitism. Thus, it is not surprising that the expected inverse relations between host condition and parasitism, commonly a premise in studies of parasite-host interactions, are inconsistently found. An understanding of how parasites encounter hosts is thus needed for developing theory for parasite-host ecological and evolutionary interactions.