[conspire] "immunity" (COVID-19, ...) Re: Numbers racket

[Rick Moen]

Quoting Paul Zander (paulz at ieee.org):

> Maybe someone the list can answer this without too much
> technicalities.  
> 
> My (miss) understanding was that most diseases are specific to one
> species.  Most animal diseases don't infect humans and vice versa.  A
> few years ago there was an avian flu that raised a lot of concern
> because it could sicken both birds and people. Why is this?
> 
> Now I am hearing stories about tigers, dogs and cats with Covid19.  I
> won't ask how the animals got tested.

To summarise the summary of the summary:  evolution.

Humans weren't the original host organisms of the pathogens that cause
measles, smallpox, influenza, HIV, diphtheria, the common cold,
tuberculosis, bubonic plague, salmonellosis, Rocky Mountain spotted
fever, Lyme disease, avian influenza, West Nile virus, rabies, anthrax, 
tularemia, Zika fever, trichinosis, swine flu, rat-bite fever, Rift
Valley fever, Lassa fever, leprosy, hantavirus, giardiasis, Marburg
viral haemorrhagic fever, Ebola virus disease, Chagas disease, and so
on, but some human molecular-biological subsystems have enough
commonalities with the usual host animal that the pathogen figuratively
says 'Hey, this is like that other place I live.  Let's move in.'

Consider avian H5N1 influenza.  If the virus stumbles into alpha 2-3
sialic acid receptors on any animal species cell, it binds to that
receptor and enters the cell.  In humans, there are a lot of these
cell-wall receptors in the gut -- or so I gather (not totally sure about
that).  If it has invaded lots of those cells before the immune system
mops up the invasion, then the resulting immune-system five-alarm
freakout induces large-scale inflammation and a cytokine storm, leading
in about half of cases to death.  Fun!  But, point is, the mechanism of 
cell entry is very non-species-isolated.

Over a very long timescale, it would be comforting (if human lives were
a lot longer, at least) to remember what Michael Crichton's character
Dr. Stone observed in _The Andromeda Strain_:  that pathogens are failed
symbiotes.  The pathogen making its host ill or dead is destroying its
home, which isn't in its evolutionary interest.  This is one reason why
some diseases that were very deadly earlier in human history have become
much less so, over hundreds of years or a thousand or so.