What factor is likely to promote the evolution of highly virulent parasites?

Highly virulent parasites are those that can cause significant harm to their hosts, potentially leading to the host's death. Horizontal transmission among unrelated hosts is a critical factor that promotes the evolution of such virulence. This transmission method allows the parasite to infect new hosts rapidly and spread widely, as it does not rely on a prolonged relationship with a single host.

In scenarios where parasites can effectively transmit between numerous hosts, there is greater selection pressure for traits that enhance their ability to exploit and exploit new hosts efficiently. This can lead to the evolution of higher virulence, as the cost of killing an individual host is less significant when the parasite can easily infect another.

Moreover, in environments where hosts do not have the opportunity for co-evolution or where they are not dependent on one another for transmission, parasites can evolve to become more aggressive and harmful without the same risks associated with co-infecting a shared host. Thus, the nature of horizontal transmission can lead to the favoring of traits associated with virulence, driving the evolution of highly virulent parasites.