III. Subtle brain germs
(Part I here, II here, IV, V to follow.)
In Part I, we mentioned 3 brain diseases where the pathogen crosses the Blood-Brain Barrier (BBB): African sleeping sickness, progressive multifocal leukoencephalopathy (PML), and bacterial meningitis. There are of course more -- polio goes from the gut to the nervous system about 1 per 100 cases. We now lay out scenarios whereby microbes affect the human brain. For simplicity, we assume that even subtle tweaking of the brain is never completely neutral to the host w.r.t. reproductive fitness, so it's + or -, w/ the magnitude to be determined case by case. We conceive of at least 4 routes for microbial influence: 1) direct, 2) indirect, 3) by chain, & 4) by inter-species cooperation -- each of which can be + or -, making 8 scenarios. While most of the cells in this matrix are unexplored, we present them to show the many ways microbes could affect the brain.
1i) Direct, Minus: The microbe makes a bee-line to its niche in the brain and exploits the brain for its own fitness. Examples: sleeping sickness, PML, bacterial meningitis, rabies; being a cat-lover (? -- admittedly small magnitude, and I say this as a cat-lover).
1ii) Direct, Plus: The microbe high-tails it to its neural niche, where it aids in cognition in order to derive some fitness benefit. Examples: unknown. But consider a microbe that chemically mimicked the effects of a neurotransmitter to increase the signal to noise ratio in neural transmission. See here for evidence that higher IQ correlates w/ lower glucose metabolization rate -- i.e., the brain expends less energy in smarter people. If a microbe helped this happen, it could, e.g., digest some of the glucose saved due to efficiency gains.
2i) Indirect, Minus: The microbe is adapted to a niche elsewhere, such as the gut or respiratory tract, but by chance one gets into the bloodstream and then into the nervous system, where it harms the host while not benefiting itself. Examples: polio; male homosexual germ (?).
2ii) Indirect, Plus: Again, a bug adapted to the gut or respiratory tract wanders into the bloodstream & crosses the BBB. Examples: unknown, though this is the category we ascribe to the "genius germs" we will posit in part V to account for very lopsided seasonality among genius births. Suppose the effect is to increase the radius R of the "spotlight" of attention that we mentioned in part II. Now the person can mentally juggle more items, boosting IQ and increasing fitness. See here (Ctrl F longevity) for evidence that higher IQ correlates w/ better health (duh). Since the bug is not specialized to thrive in the brain, it would not survive long, but a neural tweak at an opportune moment in early development could have a lasting effect.
3i) By chain, Minus: "Chain" here refers to a chain of command as opposed to an individual bug, something like an eusocial colony of microbes. Say the headquarters is in the gut but that a caste of soldiers enters the bloodstream & crosses the BBB. There, they either loot the brain for food to bring back to the hive, or they tweak the brain center for hunger to compel the host to eat more than is healthy (in any human society, hunter-gatherer or otherwise), so that more food ends up in the gut for the hive to feed on. Hunger is associated w/ the hypothalamus, a part of the brain that is easier to infect since it does not have a BBB. Examples: unknown; obesity germs (?).
3ii) By chain, Plus: Same route as 3i), though the effect would be like that of 1ii), where some of the resources saved due to efficiency gains are distributed among the hive. Examples: unknown.
4i) Inter-species cooperation, Minus: Same as 3i), though instead of specialized castes within a species, several species maximize their competitive advantage and cooperate, w/ the same effect as in 3i). Why wouldn't the species that crossed the BBB just keep all the food to themselves? Perhaps the species adapted to the gut protects them from other microbes that would otherwise wipe out the brain-adapted species. Examples: unknown; obesity germs (?).
4ii) Inter-species cooperation, Plus: Same as 3ii) but w/ the teamwork described in 4i). Examples: unknown; though boosts in IQ could require several species working in concert.
Most of the categories with unknown examples involve a benefit to the host, which probably reflects the greater likelihood of a microbe harming the host rather than helping it, as well as the innate cognitive bias of disgust -- microbes could not possibly aid cognition because they're icky, icky germs. But they must be doing something -- remember, in the gut alone there are 10^13 microbes. Let's say only one millionth of them are even capable of passing into the bloodstream and then past the BBB, and that of these potential brain germs only one percent actually does so (in only 1 per 100 cases of polio does it affect the nervous system). That still leaves 10^5 microbes, though suppose for whatever other reasons the number is cut down to 100 microbes -- they're in your body from birth. The downside to our sophisticated brains is that they take a long time to develop, leaving at least a decade for these microbes to wander in and tinker w/ this or that value for one mental module or other. So again, the likelihood that all of these microbes acting over all of this time have no effect on cognition or personality is nihil.