The virus uses it's Spike Protein to bind to the ACE2 receptor located on the surface of a cell. The Spike Protein then undergoes a conformational change which helps it to embed itself in the cell membrane, and then pull the viral membrane and the cell membrane together. Once the two membranes have fused, the virus will begin the replication process, by hijacking the cell's own structures to produce more virus.
The expression of ACE2 on cells increases greatly with age. Children and young adults have the least amount of ACE2 receptors for the virus to bind. This means the virus does not replicate well, therefore the viral load stays low which also reduces the risk of transmission. This helps explain why multiple studies such as the ones reported here, and here, have shown that instances of children passing the virus to adults in school is almost non existent.
The virus is usually spread by adults who have symptoms. Most common symptoms are headache, fatigue, loss or taste or smell, sore throat, congestion, running nose, cough, or shortness of breath. Asymptomatic spread is not well documented (see studies here, here and here). The virus is generally spread by cough, sneeze or extended contact with a symptomatic adult.
Viral load matters. For someone with no form of immunity, exposure to a high amount of virus could overwhelm the immune system. This can happen when in close contact with an uncovered cough or sneeze from an infected person. As a result the infection could gain a greater hold, and might take more effort and time to fight off.
There are interesting studies that show how many people may have had a pre-existing immunity to SARS-CoV-2. Reports of these studies can be found here, here, here and here. Past exposure to common coronaviruses may have provided a "cross immunity" for many people.
Over 114 million Americans are estimated to have recovered from Covid-19, as of March 2021 per the CDC. As of October 2021 this segment of the population with natural immunity is likely to have grown substantially.
Among those who had recovered, an estimated 4% had received some form of hospital care, according to the CDC.
Data from 800 US hospitals shows that 94.9% of people hospitalized had at least one pre-existing medical condition. Of those, 87.6% had 2 or more conditions, and half had more than 6.
Among those who have died, the total is 99.1% with at least one pre-existing medical condition. Of those, 96.5% had 2 or more conditions, and 64.2% had more than 6.
