[I'm no microbiology expert, so obviously take this with a grain of salt :-).]

I'm not sure whether the initial dose affects the severity of the disease, but it seems clear that it at least affects the probability of infection. The individual action hypothesis [1] posits that each individual pathogen has an independent chance of infecting a host, i.e.

    P(no infection | n pathogens) = P(no infection | single pathogen)^n

So if one is exposed to a single unit of the virus, infection is theoretically possible but unlikely. At higher doses infection becomes overwhelmingly likely.

I don't know how accurate the IAH model is, but there seems to be a good amount of evidence that infection rates are at least positively correlated with dose, e.g. [2].

[1] https://royalsocietypublishing.org/doi/full/10.1098/rspb.200...

[2] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2869824/pdf/120...

Well there's two things. There is the 'minimum viable dose' which you can estimate for various diseases, that will most likely result in an infection. And then there is the dose size (above that minimum).

With an exponential curve, whether you start off with 10,000 or 100,000, given that it takes about 2 weeks for our adaptive immunity to kick in, the virus is going to be in the hundreds of millions in either case.

[I work in vaccines, but not as a scientist, so I'm just trying to absorb as much as possible here..]

Yeah that's fair -- there might be some high-exposure situations (like being coughed on) where infection is very likely with or without a face shield.

I suspect that a lot of airbone infections are from inhaling small doses here and there though, considering how long mucus droplets can stay suspended [1] --

> Particles of diameters 1-3 μm remained suspended almost indefinitely, 10 μm took 17 min, 20 μm took 4 min, and 100 μm took 10 s to fall to the floor.

[1] https://www.mintie.com/assets/img/education/Aerosol%20transm...

I'm really not a medical expert at all. But, just thinking about it one could at least speculate or argue: A smaller payload means the virus takes a longer time to "become very active" in the body and delaying this "ramp up" period may give your body's immune response a longer lead time from when it first "sees" the virus to when it becomes dangerous to you.

Well with an exponential curve, even if the amount of the initial dose changes 10 fold, from 20,000 to 200,000 virons you will get to hundreds of millions in pretty much the same time. I believe adaptive immunity takes about a week or two to kick in, and in that time the virus is going to keep multiplying like crazy, regardless of the dose.

Also, suppose you were living with a partner and both of you got infected. Both of you will continually shed and "re-infect" the other, so would we expect that the infection will necessarily be severe?

There is a good New Yorker article on the matter this week:

https://www.newyorker.com/magazine/2020/04/06/how-does-the-c...