There are three types of coronaviruses that can cause severe respiratory illnesses in humans: SARS-CoV-1, MERS-CoV, and the new coronavirus SARS-CoV-2. They belong to different groups and have different receptors in the body. SARS and COVID-19 are both beta-coronaviruses that use the ACE2 receptor, while MERS-CoV is a beta-coronavirus that uses the CD26 receptor. This comparison helps us understand how these viruses work and can guide efforts in prevention, diagnosis, and treatment.