The coronavirus disease 2019 , caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), induces coughing, vomiting, or fever. The occurrence, duration, and severity of these symptoms vary from person to person, which makes symptoms of the COVID-19 infection ranging from those of a mild cold to death in severe cases. The cause of these differences can be found in individual characteristics of the patient. Age, former diseases of the patient, and they medications they took can such characteristics. As the SARS-CoV-2 affects the lung, diseases related to the lung such as chronic obstructive pulmonary disease, lung cancer, cystic fibrosis, pulmonary fibrosis, moderate to severe asthma, pulmonary hypertension, and pulmonary embolism are especially more dangerous to the human body when infected by the SARS-CoV-2. In this paper, through various methods including comparing the characteristics of mutant viruses and subunits of COVID-19, we aimed to describe the risk factors and deterioration of COVID-19 and the strain of SARS-CoV-2 during the COVID-19 pandemic.