This is my first time hearing the phrase. Google says:

> Immunity debt refers to a theoretical concept that describes a potential decline in immune system function following a period of reduced exposure to infectious diseases.

Is this the same concept you're referring to? I would hardly call that a proposterous hypothesis at face value. It's reminiscent of the generally-accepted "hygiene hypothesis", that lack of early childhood exposure to some germs causes poor immune response or even asthma later in life.

https://www.fda.gov/vaccines-blood-biologics/consumers-biolo...

This page seems focused on RSV in particular, noting that in that specific case RSV seems to not have the beneficial effect of exposure to some other thing, yet still says it may have "seemed obvious" that it should have.

> The “hygiene hypothesis” is supported by epidemiologic studies demonstrating that allergic diseases and asthma are more likely to occur when the incidence and levels of endotoxin (bacterial lipopolysaccharide, or LPS) in the home are low. LPS is a bacterial molecule that stimulates and educates the immune system by triggering signals through a molecular “switch” called TLR4, which is found on certain immune system cells.

...

> It may seem obvious that, since both the RSV F protein and LPS signal through the same TLR4 “switch,” they both would educate the infant’s immune system in the same beneficial way. But that may not be the case.

Intuitively, immunity debt is using the same logic as vaccines, that the immune system builds defenses to infections when it is exposed to them (and not before.) But I'd never heard of 'immunity debt' as a term before and I am assuming that it's promoted by people that are also anti-mask and anti-vax.

Anecdotally, I played a lot more in the dirt as a young child than any of my children, and I had much more severe asthma and allergies than they have, including the one that contracted RSV as a two-week old infant and went on to get sick just as often as their two siblings.

These are infections that people are usually exposed to so there should already be an immunity. I think what they find preposterous is the notion that the immune system needs 'exercise' in that if we don't constantly give it challenges it will get weaker, like our muscles do.