At the end of May 2020, more than 6.1 million cases of SARS-CoV-2 virus infection were registered in the world and more than 370 000 were fatal. First time outbreak of new infection occurred among residents of Wuhan, China at the end of 2019. Mortality rate for the current COVID-19 epidemic is significantly lower than for severe acute respiratory syndrome (SARS) or Middle East respiratory syndrome (MERS). However, the SARS-CoV-2 virus spreads much faster than SARS and MERS and causes far more deaths totally than both SARS and MERS combined. Information on factors that affect the development, course and outcome of infections caused by coronaviruses stated in the article was obtained from investigations carried out during the period of previous epidemics of coronavirus infections and during the current SARS-CoV-2 pandemic. The following factors are discussed in the present article: the direct cytopathic effect of viruses, infection of the immune system cells, the role of inflammation in the development of the disease, innate immunity factors, autoimmune reactions, features of the expression of immunoregulatory molecules, the role of host organism factors, including genetic ones.