Viruses, unlike bacteria, cannot replicate without access to the host’s cells. This means that each individual virus can only infect a single cell, producing hundreds or thousands of copies of itself before the cell undergoes necrosis (non-programmed cellular death), rupturing and expelling the product virions, some of which will go on to affect other cells. The host’s immune system cannot ‘see’ inside the virus inside the cell but the cells produce class I major histocompatibility complex proteins (MHC class I) which will display fragments of protein from the virus upon the cell surface. The host’s immune system produces cytotoxic T-cells which recognize a particular antigenic peptide bound to an MHC molecule, ‘informing’ the immune system of the presence of a pathogenic invader. The T cell releases cytotoxic factors to kill the infected cell which prevents viral replication and release of new virions. The immune system will also develop antigen-specific antibodies, which essentially tackle the pathogens, deactivating and removing them from the body. (There are also intrinsic proteins in the cells called interferons, which deactivate the virus before it can replicate and prompt the host’s immune system, but these don’t work against novel viruses; these are the reason you can be exposed to millions of different types of viruses every day and not be infected.)
It takes time for the host immune system to respond to an new infection; several days to a couple of weeks in most cases, so the higher viral load means more infected cells, and therefore more replications in geometric scale prior to developing immune response, and such a mass of infected cells can overwhelm the host immune system’s ability to destroy virions faster than they are produced. So, having a few infected cells may be just enough to provoke an immune response without developing any evident signs or symptoms, but a massive viral load may overwhelm the immune response of even the most healthy adult. Child, whose immune systems are running at hyperspeed all the time because every pathogen is essentially novel to them, are able to control most infections, hence while the so-called ‘childhood diseases’ like chickenpox and measles are essentially just an irritation for them but can be lethal or have serious and long-lasting consequences for adults.
It is also the case that doctors and nurses, who are in essentially intimate contact (e.g. direct mass contact with droplets or aerosols containing large quantities of virions) are getting exposures deeper within the respiratory system (deep within the pleura of the lung) which causes a much faster and more severe onset of the disease than infection from incidental contact that may only reach the trachea. This results in a faster and more severe onset of the COVID-19 disease before the immune system can even begin to respond, and thus, a more severe progression and greater potential for pleurisy (inflammation of lung tissues), pleural effusion (fluid leaking into the space between the lung and chest wall), or interstitial lung disease (fibrosis) as well as the potential for bacterial co-infection leading to bacterial pneumonia (bacteria colonizing in the lung requiring antibiotic treatment) instead of the more mild bronchial infection resulting in only induced asthma and acute bronchitis in most patients.
There is increasing evidence of aerosol transmission of the SARS-CoV-2 virus, which would explain how rapidly it is able to spread with even casual (non-intimate) contact, but while causal contact will only result in intermittent exposures to aerosols or fine droplets, doctors and nurses working in an enclosed environment with multiple patients are essentially surrounded by the virus at all times. This overwhelms the normal protections and procedures used in hospitals (which are essentially the equivalent of Biosafety Level 2 in operating and exam rooms, and nothing in waiting areas and other areas of the hospital), and since medical personnel are now having to reuse one-use protective equipment such as respirator masks it means that exposure is almost certain as they repeatedly re-don contaminated gear. At this point, the masks are essentially hopeful pleading to prevent infected medical personnel from spreading the contagion to uninfected patients, although again given the infectiousness of this pathogen that may be futile.
So, yes, a higher initial ’viral load’ can lead to faster, more acute, and potentially more life-threatening progression of viral infection and disease, and people in hospitals are significantly more likely to be exposed to both large amounts of virions and in a form (aerosol or fine droplets) which is more likely to be inhaled deeply into the lungs where the infection can do more damage before the host’s immune response is prompted.
Stranger