Parthenogenesis is the development of a female egg into a new individual without the contribution of a male set of chromosomes.
In a hypothetical pond, two populations of fish are found; one reproduces through sexual reproduction, the other through parthenogenesis. Initially, the parthenogenetic fish are a larger population when the pond is first studied. However, researchers soon observe that the parthenogenetic fish are more frequent hosts for a new blood-borne parasite than the sexually reproducing fish.
As a result, the sexually reproducing population increases and the parthenogenetic population decreases.
Which of the following best explains the reason for the shift in population numbers?