The average person breathes about 16 times a minute and transfers about 4.8 litres each breath. That’s about 76 litres of air a minute. Figures of 200mph have been assumed for coughs and sneezes for many years, but recent testing show that those figures were nonsense and never properly researched or doubted, just repeated parrot fashion. The figure is closer to 10mph. The average breath is around 4mph at the mouth. The average aerosol droplet is between 10-1000µm, the larger the droplet the less time it will stay in the air, and the less distance it can travel. The average size is around 100µm, which is the likeliest for infection, and will stay in the air about 6 seconds on a downward curve from the mouth. 1000µm will stay in the air about 0.6 seconds, and 10µm 10 minutes. But the 1000µm droplet will have 1,000 times the viral content of the 100µm one, the 10µm will probably have 1,000th of the viral content of a 100 µm one. You may or may not get lucky but the averages are such.
Also probably about 95% of transmission is aerosol.
If you take the intake of a person of 4.8 litres around them the droplet needs to be within about 0.3 metres of the mouth. But it’s a statistical exercise of percentages as there are so many variables. One of the key ones being direction.
So a short woman or especially a child needs to be farther away from a tall man.
Normally this mean that between two similar height people roughly face to face most of the content will have dropped below the acquisition level after about 6 feet.
For a 6 foot man to a 5 foot woman, that would be closer to 8 feet. For a 5 foot woman to a 6 foot man that would be standing next to him.
For sneezes and coughs you need to double the distance. Of course if you’re bending down the distance figure increases.
Also the smaller the area you are in the more likely you will stay in a closer proximity, a car being the worst and sitting in the front very bad.
Being in the open air the best, especially on a sunny day.
Similarly on a bus, the farther back you are the better, the closer to the front the worse, but not so good on one entrance ones.
As for masks it depends on it you wear them properly. The reduction risk if you don’t is probably only about 30%, but if you do probably around 95%.
Internally the disease is controlled by surface epithelial cells both external and internal, using the ACE2 content. When you are young your cells are growing in their early regenerations and slows down into homeostatic levels around 20. During the next 20 years they stay in that sage until the regenerations make them less flexible and efficient. From then they thin out, noticeable in he skin and the imperfect regeneration and repair. This is what the Covid-19 attacks, cells death allowing the virus to transfer more easily around the body, but carrying a passenger, so when they are regenerated they start to do it pre-infected. The more damage that is caused initially, the greater the repair and transfer in an avalanche fashion. But you don’t try to stop an avalanche when it’s half way down a mountain, you do it at the top. This is where immediate testing and the use of something like remdesivir would be useful.