Let me without going into more academics give a brief of what I learnt at medical school of body’s defence mechanisms. Before you feel sick, your body had tried to fight the infecting pathogen through a variety of defence mechanism. These are classified as primary or secondary mechanisms. Primary mechanisms prevent entry of pathogenic organisms-what is commonly known as germs-into the internal tissues. Secondary mechanisms destroy organisms that manage to enter into the tissues.
In one of primary mechanisms is reflex action: Where coughing, sneezing and vomiting are reflex actions that remove foreign materials from the respiratory and digestive tracts-and that what happens when covid-19 patient cough or sneeze and in process shed coronavirus into air which infect others if they inhale it.
However, the recipient need a high dose to overcome secondary mechanisms in what is termed as pathogenicity or virulence level. And here is where ventilation comes in to defuse and dispersion of germs-carrying particle to prevent concentration.