>> Also, many virologists mentioned in recent texts that the initial concentration of the virus you receive can affect how sick you'll get - the more viruses you're exposed to, the faster they can invade the body and the more severe it will get.
That is basic infections 101. When you are exposed to any dangerous virus a race starts between the virus and your immune system. If the virus starts out only infecting a handful of cells, your immune system has a head start in developing antibodies before symptomatic infection sets in. (This is also a basic principle behind many vaccines.) But if you are hit will a massive viral load that instantly infects every cell in your lungs, the immune system is fighting uphill from day one. A massive initial viral exposure can also trigger an excessive immune response, for instance dangerously high fever. Such an immune response can be as deadly as the virus. Much covid research is going into not defeating the virus directly but regulating/slowing the immune response to the patient survives their own immune response.
This principal explains why healthcare workers are suffering so. They are exposed to constant massive doses of virus, possibly from multiple patients carrying slight different versions of the virus. So they get sicker than people who are exposed in the general community.
There's already a comment above pointing out that it might be the initial infection location plays big role. Seems very likely that poor ventilation (in e.g. medical facilities) is main cause of severe cases.
Also, the virus is replicating exponentially only if it can reach many uninfected cells. It takes ~10 hours for an infected cell to start producing virus. Not sure whether non-specific immune system can somehow "contain" virus, would be great to learn about that.
That is basic infections 101. When you are exposed to any dangerous virus a race starts between the virus and your immune system. If the virus starts out only infecting a handful of cells, your immune system has a head start in developing antibodies before symptomatic infection sets in. (This is also a basic principle behind many vaccines.) But if you are hit will a massive viral load that instantly infects every cell in your lungs, the immune system is fighting uphill from day one. A massive initial viral exposure can also trigger an excessive immune response, for instance dangerously high fever. Such an immune response can be as deadly as the virus. Much covid research is going into not defeating the virus directly but regulating/slowing the immune response to the patient survives their own immune response.
This principal explains why healthcare workers are suffering so. They are exposed to constant massive doses of virus, possibly from multiple patients carrying slight different versions of the virus. So they get sicker than people who are exposed in the general community.