The virus first enters the nose and throat and makes way into the cells. Here, they hijack the cell's machinery and starts multiplying within them. https://twitter.com/Ayshmarq/status/1319933222042636288

As they multiply within, they begin to release millions of copies of viruses inside the body, invading more and more cells. Usually, this takes a few days which is where the symptoms of fever, dry cough, sore throat. loss of smell and taste start appearing.

if the immune system does not overpower the virus, the virus will usually go down the windpipe and start attacking the lungs. Inside the lungs are structures called alveoli, which are great resources for viral replication. This is where the virus will now choose to replicate.

Now. alveoli are where oxygen transfer occurs. When the immune system starts battling the invader at the alveoli disruption to the oxygen exchange process begins as well. The more reinforcement needed the more pus and fluid that starts accumulating within the lungs.

Some patients will recover from this point, but some will need oxygen to help them through this process. But if the infection becomes worse, patients will develop what we call ARDS- acute respiratory distress syndrome, causing O2 levels to deplete eventually causing death.

Some suggest a phenomenon called a "cytokine storm" an overreaction from the immune system that triggers and is responsible for patients' death.

Apart from this pathway, the virus seems to be able to strike other organs too. These include the heart, kidney, brain, and central nervous system.

Based on these, the NHS has two levels of risk. High and moderate. High-risk patients, who are severely immunocompromised (eg: pt who had an organ transplant, chemotherapy, serious heart conditions)

Moderate risk patients have as the term suggests moderately immunocompromised. Eg: >70years, diabetes, heart disease, very obese or pregnant.

That is the relationship between COVID19 and pre-existing conditions.