immune—before they reached childbearing age. From a strictly evolutionary perspective, though, vertical transmission is not a strategy upon which rubella virus could depend for long-term success. A miscarried fetus or a blinded baby with heart troubles will most likely be a dead-end host, just as terminal for the virus as a Congolese nun with Ebola.
Whatever mode of transmission a virus favors—airborne, oral-fecal, blood-borne, sexual, vertical, or just getting itself passed along in the saliva of a biting mammal, like rabies—the common truth is that this factor doesn’t exist independently. It functions as half of that ecological yin-yang.
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And the other half, virulence, is more complicated. In fact, virulence is such an iridescent, relativistic concept that some experts refuse to use the word. They prefer “pathogenicity,” which is nearly a synonym but not quite. Pathogenicity is the capacity of a microbe to cause disease. Virulence is the measurable degree of such disease, especially as gauged against other strains of similar pathogen. To say that a virus is virulent almost sounds tautological—the noun and the adjective come from a single Latin root, after all. But if “virus” hearkens back to “poisonous slime,” the point of virulence is to ask, How poisonous? The virulence of a given virus within a given host tells you something about the evolutionary history between the two.
Just what does it tell you? That’s the tricky part. Most of us have heard an old chestnut on the subject of virulence: The first rule of a successful parasite is Don’t kill your host. One medical historian has traced this idea back to Louis Pasteur, noting that the most “efficient” parasite, in Pasteur’s view, was one that “lives in harmony with its host,” and therefore latent infections should be considered “the ideal form of parasitism.” Hans Zinsser voiced the same notion in Rats, Lice and History, observing that a long period of association between one species of parasite and one species of host tends to lead, by evolutionary adaptation, to “a more perfect mutual tolerance between invader and invaded.” Macfarlane Burnet agreed:
In general terms, where two organisms have developed a host-parasite relationship, the survival of the parasite species is best served, not by destruction of the host, but by the development of a balanced condition in which sufficient of the substance of the host is consumed to allow the parasite’s growth and multiplication, but not sufficient to kill the host.
It does seem logical, at first consideration, and it’s still often taken as dogma—at least by people who don’t happen to study the evolution of parasites. But even Zinsser and Burnet, to their credit, hedged their endorsements of this idea. They must have recognized that the “rule” was just a generalization with important, revealing exceptions. Some very successful viruses do kill their hosts. Lethalities of 99 percent, and persisting at that level over time, aren’t unknown. Case in point: rabies virus. Case in point: HIV-1. What matters more than whether a virus kills its host is when.
“A disease organism that kills its host quickly creates a crisis for itself,” wrote the historian William H. McNeill, in his landmark 1976 book Plagues and Peoples, “since a new host must somehow be found often enough, and soon enough, to keep its own chain of generations going.” McNeill was right, and the key word in that statement is “quickly.” Timing is all. A disease organism that kills its host slowly but inexorably faces no such crisis.
Where’s the balance point in that dynamic interplay between transmission and virulence? It differs from case to case. A virus can succeed nicely in the long term, despite killing every individual infected, if it manages to get itself passed onward to new individuals before the death of the old. Rabies does that by traveling to the brain of an infected animal—commonly a dog, a fox, a skunk, or some other mammalian carnivore, with flesh-biting habits and sharp teeth—and triggering aggressive changes of behavior. Those changes induce the mad animal to go on a biting spree. In the meantime, the virus has traveled to the salivary glands as well as the brain, and therefore achieves transmission into the bitten victims, even though the original host eventually dies or is killed with an old rifle by Atticus Finch.
Rabies also occurs sometimes in cattle and horses, but you seldom hear about that, probably because herbivores are less likely to pass the infection along with a furious bite. A poor rabid cow may let out a piteous bellow and bump into a wall, but it