Desperately seeking hypovirulence
A contagious pathogen that kills its host too quickly is a failure.Even though hypovirulence may be in the long-term best interest of the species, hypovirulence is usually not in the short-term interest of a given strain of that pathogen.
It is one of the classic contradictions to Adam Smith's Invisible Guiding Hand theory which holds that autonomous actors will always arrive at a global optima as an unerring side effect of seeking their own, personal optimum.
The pathogen may end up at that place but the path is messy and strewn with wreckage.
Hypovirulent strains cannot compete with the speed and totality that the hypervirulent strains exhibit when exploiting the nutrient resources of the host.
It is the compound interest problem, again. If aggressive exploitation (which kills the host) gives the hypervirulent strain a 2X advantage, the hypervirulent strain will outnumber the hypovirulent strain by a factor of one million-to-one after 20 generations.
The pathogen lays vast populations to waste. Tiny, isolated pockets of the host remain and pump seed out into the void. The pockets grow until they encounter a smoldering ember of the pathogen and the process repeats.
Hypothetically, one of those smoldering embers will be less virulent than the first wave. Its victims will be able to survive the infection and the pathogen will be able to undergo multiple generations of spoor release without killing the host. That multigeneration spoor release and the sparse distribution of the host is the basis for the hypovirulence's competitive advantage over the hypervirulent strains.
Another way to say the above is that hypervirulent strains will dominate when the hosts are close together and speed of exploitation (invasion, extraction) is the bottleneck in reproduction. Hypovirulent strains will have a slight selection advantage when discovery of new hosts is the primary bottleneck in reproduction. This state may oscillate back and forth as population density changes and climatic factors vary---either favoring or suppressing the ability of the pathogen to "find" new hosts.
So, where might one look for hypovirulence?
The hypothesis presented here suggests one might find hypovirulence in two, very different places.
One place to look is the pathogen's epicenter where, presumably, the host and the pathogen have had the longest amount of time to oscillate their way to an equilibrium. This is where the host population was first decimated and where the selection pressure first shifted to "host discovery".
When the Varroa Mite started devastating honey bee hives, researches went to the Amur region of Siberia (north of Korea) to collect resistant strains of honey bees. Their focus was on finding resistance in the host rather than finding friendlier pathogens. But the same principle applies.
The second place to look are at the fringes of the host's natural range where the pathogen is petering out due to the scarcity of the host. The primary selection pressure on the pathogen is "host discovery" and hypovirulent strains should become relatively more abundant.
Presume that the host's range covers an area large enough to show climactic variation. Also presume that one side of the range favors the spread of the pathogen. It follows that the pathogen will spread on the less friendly side only in those rare years when the weather favors the pathogen. That amounts to an extreme penalty against the one-and-done, hypervirulent strains and creates a relative bias for the hypovirulent strain.
Therefore, if a pathogen favored cool, humid conditions then one might be well advised to look for hypovirulent strains along the southwestern (hotter, drier) fringe of the host's distribution.
Desperately Seeking, Part IV