Lung bronchiols have tiny cells called alveoli. The airways of alveoli are flexible. Small blood vessels called cappilaries surrounds alveoli. Oxygen from air you breathe passes into capillaries and carbon dioxide from body passes out of capillaries into alveoli so that your lungs get rid of it when you exhale. Through airways, most of the germs get catched in mucus that lines the trachea. Hairs like cilia constantly push the mucus and germs out of your airways where it get expell from your body through coughing and sneezing. The cells of your immune system attack viruses and make it passed through mucus in cilia.
However if your immune system is weakened as in case of coronavirus infection, the virus can overwhelmed your immune cells. Bronchiols and alveoli get inflamed as your immune system attack to multipling viruses. This inflammation can cause your alveoli to flow a fluid making it difficult to your body to get oxygen you need. Thus develop pneumonia which cause difficulty in breathing, chest pain, headache, coughing, fever and chills etc. This condition is also fetal when untreated.