Attempt at a gears model of getting infected with COVID-19

My understanding of the infection process for viruses (such as SARS-COV-19) is that when one or more viruses get past the physical defenses (mucous, stomach acid, etc) they inject their RNA payload into a cell, and produce more copies of the virus. This is an exponential growth process, dealing exponential damage to the body with time.

Of course, having 100 cells out of trillions get infected and killed is completely unnoticeable, but if the infection grows to millions, that starts to have an impact.

The body then has a step-wise defense. First there are general purpose immune response cells that catch and kill generic intruders. So, if you literally had one copy of a virus get in, it (or it's children or grandchildren) would just get rounded up and disposed of without any impact. This happens all the time, as there are viruses everywhere and we are constantly bombarded by a low level of self-replicating intruders.

If the initial (or repeated) exposure generates enough of the same virus, the body creates a set of special response cells (T-cells), which provide a super-exponential response (the T-cells are not consumed as they dismantle viruses, and they replicate exponentially), which allows the immune system to catch up to the viral growth and  get rid of it.

In some cases a Fever or other bio-kinetic response can make your body a less hospitable environment for the invader.

The general malaise and "feeling sick" comes from a combination of your cells dying to the virus, and your body re-directing energy away from normal maintenance & activity to produce enough immune response cells to catch up to the exponential growth.

There's room to expand on the bit about how live virus particles might reach your respiratory system:

• Physical contact between mucosa and contaminated surface. The degenerate case of this would be if someone wiped their hand along a surface that an infectious person had coughed on and immediately picked their nose: droplets could get from the infected person to the mucosa of the person with poor hygiene without the latter technically inhaling them.

• Eye contact. A literature review from October on the subject seemed inconclusive, but can't rule it out. There's also the much-talked-about paper on lower COVID rates among glasses-wearers. Since tear ducts drain into the nose, the "stuff that gets in your eyes ends up near your respiratory system" link seems plausible enough to take seriously until better research on the topic becomes available.