Vertical or horizontal transmission of the parasites

In an external parasite, transmission may mean the movement of an adult parasite that has been living off one individual host on to another host; in internal parasites it typically means the movement of the offspring of parasites living inside one host on to another host.

Vertical transmission: a parasite transfers from its host to the offspring of that host; it can be by a variety of mechanisms - by the mother’s milk, or simply by jumping from host parent to host offspring when the two are near each other, or inside the gamete.

Horizontal transmission: the parasite transfers between unrelated hosts, and not particularly from parent to offspring; transmission may be by breathing, or by a vector such as a biting insect, (malaria passed by mosquitos like this one opposite is a good example).

Some parasites are transmitted vertically, others horizontally: what consequence does this have for the evolution of virulence?