The S protein of SARS-CoV-2 binds to ACE-2 receptor in lungs. This receptor is expressed in gastrointestinal tract, heart and, most importantly, type 2 alveolar epithelial cells. Once the virus gains entry into the human lung cell, the serine protease TMPRSS2 in host cell cleaves the S protein in SARS-CoV-2. This results in fusion of cellular and viral membranes. The type 2 alveolar epithelial cells serve as reservoir for viral replication while the large surface area facilitates the replication and invasion. When SARS-CoV-2 initiates lung injury, the body responds by releasing inflammatory mediators i.e. cytokines and TNF-alpha. The immune response serves as deteriorating factor in COVID-19 infection.