VIRUSES ARE NOT ALIVE.
THEY ARE PARCELS OF GENETIC MATERIAL WRAPPED IN A SHEET OF PROTEIN.
THEY CAN HIJACK AND TRAVEL BETWEEN OUR CELLS.
All living things are built from a genetic pattern carried in each cell.
To grow, cells need to copy that pattern together to make tissues. Tissues combine together to make organs. Organs together make systems, and systems together make individuals.
This growth requires energy - and in animals, that energy comes mainly from chemicals burnt with oxygen. Oxygen is harvested from the outside environment by lungs.
Any system contacting the outside is protected and covered by skin. The skin of the lungs, called respiratory epithelium, is special because of that crucial oxygen harvesting role, and is essential for life of the animal.
The lungs reaps oxygen from their deepest parts, so they have a protective mechanism nearer to the outside called the mucous escalator.
This epithelium secretes mucous to its outer surface and grows little hairs (cilia) which rhythmically push this mucous up and out of the lungs. The mucous is then coughed up as sputum in the mouth or appears at the nostrils - commonly known as snot.
All dust and contaminants are trapped on this mucous escalator and are thus removed from the lungs.
Skin is a barrier; the first and most important line of defence against infection. All skin (epithelial) cells wear out (including respiratory ones) quite quickly. They are constantly being replaced, requiring millions of gene copying procedures every day.
Nothing is perfect and sometimes a mistake occurs in this copying process; a mutation.
Most mutations make that new cell die... but very occasionally a mutated cell may continue to live with a different function.
There are two types of new function that threaten the individual. One type is when the new cell goes rogue - it copies itself rapidly and ceases to perform its normal function. These simply grow and grow and take over from the normal cells. This is called a tumour or cancer.
Viruses evolve similarly.
This time, instead of cells growing themselves uncontrollably, a virus-infected mutant cell manufactures packages of genes which make the virus able to replicate itself further. This also happens uncontrollably, and the cell fills with so many virus packages that it bursts and releases them into the body.
These virus packages are made with the same (or similar) protein wrapping as the cell from which they evolved, they are able to lock on to similar cell clones nearby.
Before long the skin of the lung is bursting with virus particles, no longer properly preforming its crucial role. This leaves holes in the skin (ulcers) and stimulates the second line of defence: inflammation with all its extra fluid.
The mucous escalator becomes very irritated and starts to fail. The body reacts with prolonged coughing and sneezing in an attempt to eject the irritant-filled mucous from the lungs. Ulcers allow bacteria to enter the body, triggering further inflammation and fluid - an subsequent army of white blood cells arrive to battle these bacteria. Further fluid escapes in to the tubes of the lungs, and the individual can then drown dead if the coughing and sneezing is unable to clear the airways. This is pneumonia.
Needless to say, coughing and sneezing not only carries out excess mucous, but also millions of the tiny virus particles in an aerosol bloom, or spray, outside the body.
These particles then settle on any surface in the immediate environment - such as hands, elbows, clothes, bench tops, doorhandles, cutlery etc.
Infected particles also accumulate on the patient's face, where fingers often itch and scratch. All of these external objects and surfaces become vectors.
Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000. Section 6.3, Viruses: Structure, Function, and Uses.
John H. Humphrey and Samuel Scott Perdue; Immune system,
Encyclopædia Britannica. March 04, 2020
Aparna Vidyasagar. What Are Mutations? Live Science. January 14, 2016
Anthony J.F. Griffiths, Mutation, Encyclopædia Britannica. March 16, 2020;
Viral evolution; Primordial cellular origins and late adaptation to parasitism; Arshan Nasir, Kyung Mo Kim, 3 and Gustavo Caetano-Anollés; Mob Genet Elements. 2012 Sep 1; 2(5): 247–252.