Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

From a general evolutionary standpoint, another phenomenon is very interesting and apparently determines to a substantial degree the result of the evolutionary battle between a parasite and its host. The two participants in the co-evolutionary process, here the parasite and its host, are not concerned to the same degree with the result of mutual interactions. While losing the battle with the host organism generally leads to death for the parasite (as an individual), for the host it generally leads only to a greater or smaller reduction in its fitness (Dawkins & Krebs 1979; Dawkins 1982). The fact that the host organism is sometimes killed by the parasite or is not able to reproduce and its fitness thus decreases to zero does not much affect the situation. This is not a typical situation as it is generally in the interest of the parasite not to kill its host and thus the relevant selection pressures to which the parasite is exposed mostly lead to a gradual reduction in its pathogenicity to a certain optimal level (see XIX.5).
The “life-dinner” principle is valid not only in the relationship between a parasite and its host, but also in a great many other inter-species and intra-species interactions. This principle was originally identified (and named) in systems of the predator and prey type. Put simply, rabbits run faster than foxes for the simple reason that they are running for their lives, while the fox is only concerned about its supper.