In what type of parasite is decreased virulence expected to evolve?

Decreased virulence is expected to evolve in vertically transmitted parasites because these parasites have a unique relationship with their hosts. In vertical transmission, parasites are passed from parent to offspring, which often leads to a more intimate association between the parasite and the host. This close relationship creates a scenario where extreme virulence could be detrimental to the parasite's own reproductive success, as overly aggressive exploitation of the host could harm the host's health and fitness, ultimately reducing the chances of the parasite being transmitted to the next generation.

When a parasite is too virulent, it risks killing its host or severely impairing the host's ability to reproduce, which can significantly limit the parasite's own transmission opportunities. Therefore, natural selection favors lower virulence in vertically transmitted parasites, as moderate levels of virulence can maintain host health to a degree that facilitates continued transmission.

In contrast, horizontally transmitted parasites, which spread between hosts through various means (e.g., direct contact, vectors), may not face the same selective pressure to reduce virulence. They can afford to be more aggressive because they do not rely on maintaining a long-term relationship with a single host for reproduction. Environmentally persistent parasites may also not evolve lower virulence in the same way since their transmission dynamics are different. Infect