Coronaviruses are enveloped, helical nucleocapsid viruses with a size of approximately 120 nm, containing single-stranded, positive-sense RNA (mRNA). They have the largest known RNA genome (approximately 30 kilobases) among all viruses. Coronaviruses cause infection in many different animals including but not limited to, pigs, cats, rodents, bats, as well as humans. Human coronaviruses (abbreviated as HCoV) belong to the Coronavirinae subfamily.
Human coronaviruses were first detected in patients with the common cold in the 1960s. Coronaviruses are transmitted through the respiratory tract and primarily affect the upper respiratory and gastrointestinal tract of mammals and birds. So far, 7 strains of coronavirus have been shown to affect humans (229E, NL63, OC43, HKU1, SARS-CoV, MERS-CoV, and SARS-CoV-2).
Until recently, the most known human coronavirus (SARS-CoV) caused the SARS (severe acute respiratory syndrome) epidemic, which was seen in South Asian countries between 2002-2004. On the other hand, SARS-CoV-2 was first detected in Wuhan, China at the end of 2019, and then quickly spread to different countries. It was defined as a pandemic, that is, a global epidemic, by World Health Organization (WHO) on March 11, 2020. The illness was initially referred to as the Coronavirus Disease 2019 (COVID-19), and it has now emerged as the coronavirus with the greatest level of public awareness.