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by Kinoshita Yoshikazu (pseudonym)
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Epistemological status: I believe this is wrong, but I would like to know why

I have this question because the conclusion

--"If the human society do not have access to medical interventions, the burden of infectious diseases would increase over time as more types of pathogens are established in the population"

seems to follow from the below assumptions (I think most of them are correct, maybe with caveats): 

1: Smallpox is the only (major?) infectious disease that we have eradicated. It would be an impossible feat in the absence of modern healthcare, and replicating the success with other infectious diseases would be more difficult or straightaway impossible.

2: New infectious diseases become established in the human population overtime through zoonosis (and maybe other pathways too)

3: Being infected by pathogen A confers some immunity to A but does not offer protection to infection by pathogen B. 

 

Combining all 3, it seems to me that as more types of pathogens become established in the human population, the burden of infectious disease on each individual will increase. 

As an oversimplified example, suppose that immunity of influenza does not protect an individual from infection by SARS-covid-2 and vice versa. 

Prior to 2020, there is a "flu season" each year as the population's immunity to influenza waxes and wanes. 

If we did not implement any public health measures during the covid pandemic, the spread of influenza would continue unchanged due to the absence of cross-immunity.

Then after covid reached a steady state in the human population (perhaps a seasonal pattern), each year a human individual will have to contend with "SARS-covid-2 season" on top of "the flu season" (which continues at the same level as pre-2020), and this I believe means an increase of the burden of infectious diseases on humanity. 

I think I'm missing something here and this conclusion is wrong, but I can't tell why.

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JoshuaBlake

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One thing you're missing in this model is that they'd be selection pressure on humans to develop stronger natural immunity or ability to live with these pathogens.

It's also a weird counterfactual since it's plausible that technology enables infectious diseases (due to human density and connectivity) in the way that we experience them. It seems that pandemics may have been especially bad ~19th century when the world was quite connected but there was no modern medicine (germ theory, antibiotics, etc.).

Kinoshita Yoshikazu (pseudonym)1
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That is true! Maybe the disease burden increases when new pathogens are introduced but eventually reaches a balance.

But it doesn't seem to completely remove the effect from accumulating multiple pathogens in the community, a population with covid+flu (or any other combination of pathogens with no cross-immunity) in circulation and adapted to live with them will probably still have a higher disease burden than a population with just the flu in circulation and adapted with them.

Quite likely that the 19th century was worse than before due to increased population density and global connectivity, and I shudder to imagine what if AIDS became established in humans in the 19th century instead of the 20th. 

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Linch5
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Some additional reasons in favor of this hypothesis:

  • greater degree of interconnectedness and travel
    • compared to the past, a new zoonotic disease in Vietnam is much more likely to Brazil, and vice versa.
  • (disputed) factory farming increases both the absolute number of sick animals, and human-animal transmision, and factory farming appears to go up with time
  • (high uncertainty) empirical rate of epidemics seems to be going up with time 
  • (possibly) climate change
  • lab accidents/escape and other artificial causes of novel pandemics

Some reasons against:

  • adaptive immunity, especially in youth. If young people are exposed to H5N1, SARS-CoV-2, etc, they a) are in many situations less directly affected by eg covid and b) can build long-lasting immune responses such that future exposure to them as adults will be less effective than on immunologically naive adults.
  • evolution of pathogens to be less lethal with time
    • While this is by no means guaranteed, under some models there are tradeoffs between how lethal a pathogen is, and how likely they are to infect people. To the extent this is true, we might expect formerly lethal pathogens to become less lethal with time
      • This is hypothesized to have occurred with the Spanish flu and earlier (pre-COVID, pre-SARS) coronaviruses
  • social/policy responses to the threat of pandemics
    • in the absence of advanced medical interventions, we might still have social/policy non-medical interventions to try to reduce the load of pandemics.
    • for example, flying might become less common (more penalized), quarantines might become more common

On balance, I personally think it's more likely that we'll have increased rather than decreased probability of pandemics in the coming decades, especially in the absence of next-gen countermeasures. But I'm pretty uncertain overall, and I can imagine it flipping either way.

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Kinoshita Yoshikazu (pseudonym)1
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I think the second reason "against" is probably the only true argument that my hypothesis is wrong.

The first reason still doesn't prevent the disease burden after covid/any new pathogen spilled into humans from being appreciably bigger than the disease burden before the event. 

The third reason is of course about interventions, which can go...many ways.

I didn't raise it in my original question but I wondered if this hypothesis applied to non-human species, which would still be a pretty interesting problem since it might impose a limit to the propagation of any species (as it collects more different kinds of pathogens overtime and need to contend with them). 

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