One of the major concerns in the world is the Coronavirus Disease 2019 (COVID-19), an infectious disease caused by Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2). Although the knowledge is limited about the risk factors for the disease, morbidity and mortality may increase in the patients with some underlying conditions such as cardiovascular disease, hypertension, diabetes, cancer, etc. These diseases can weaken the immune system and affect the body's ability to respond to infectious agents. Therefore, these patients are more at the risk for COVID-19 and also the underlying condition may worsen the severity of COVID-19 infection. On the other hand, SARS-CoV-2 via multiple pathophysiological mechanisms can lead to the progression of the underlying diseases and resulting in a poor outcome. The coronavirus binds to angiotensin-converting enzyme 2 (ACE2) which is expressed on the cells of many organs and it can directly affect tissues. Apoptosis of the cells may occur in patients with acute respiratory disease syndrome (ARDS) due to hypoxia in COVID-19. Moreover, SARS-CoV-2 leads to an imbalanced the immune inflammatory response in some patients which may cause indirectly organ injury. In this review we described the prevalence of COVID-19 in various underlying diseases and the impact of the SARS-CoV-2 in the outcome of these diseases. However, further studies are needed to investigate the prevalence of this new virus in patients with underlying diseases and its effects on the progression of illness.