https://youtu.be/EIzu8yvr6S8 Coronaviruses are a group of related RNA viruses that cause diseases in mammals and birds . In humans, these viruses cause respiratory tract infections that can range from mild to lethal. Mild illnesses include some cases of the common cold (which is also caused by other viruses, predominantly rhinoviruses ), while more lethal varieties can cause SARS , MERS , and COVID-19 . Symptoms in other species vary: in chickens, they cause an upper respiratory tract disease , while in cows and pigs they cause diarrhea . There are as yet no vaccines or antiviral drugs to prevent or treat human coronavirus infections. Coronaviruses constitute the subfamily Orthocoronavirinae , in the family Coronaviridae , order Nidovirales , and realm Riboviria .They are enveloped viruses with a positive-sense single-stranded RNA genome and a nucleocapsid of helical symmetry. The genome size of coronaviruses ranges from approximately 26 to 32 ki