The answer is spread/dispersal of the parasite. They
increase the probability of infection of the definitive host. The intermediate
host can either be present or absent in the parasitic life cycle. They are
considered an ecological than a
physiological phenomenon for the transmission of the parasite.
Eutrophication, the process by which a body of water acquires a high concentration of nutrients, has become known to the environmentally aware community as a key human stressor on the world's ecosystems. High concentrations of nutrients, specifically phosphates and nitrates, promote excess growth of primary producers.
Visit the website : https://www.nownovel.com/blog/what-is-story-setting/ or
https://www.enotes.com/homework-help/why-do-authors-make-setting-character-shifts--388343
Hope it helps
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