Which condition would predict that higher virulence of a parasite might evolve?

The higher virulence of a parasite is more likely to evolve under conditions of horizontal transmission between hosts. In horizontal transmission, a parasite spreads from one host to another, often without the need for a prolonged association with any single host. This mode of transmission can favor increased virulence because the parasite can heavily exploit its current host's resources before it potentially kills or severely harms it, as it can readily move on to infect new, uninfected hosts. With this dynamic, the rapid spread and transmission of the parasite can lead to selection for traits that enhance its ability to infect and manipulate its hosts, resulting in higher virulence.

In contrast, mutually beneficial host-parasite interactions would promote lower virulence as both the host and parasite benefit from the relationship, fostering a balance that mitigates the harmful impacts of the parasite. Long-term host associations would also typically select for lower virulence since a parasite that kills its host too quickly would risk its own extinction, as the host may not survive long enough to spread the parasite to new hosts. Increased genetic diversity in the host population could potentially lead to better resistance against parasites, which would reduce selection pressures for high virulence in the parasite.