SARS-CoV-2 is an enveloped RNA β-coronavirus genetically similar to the virus that causes Severe Acute Respiratory Distress Syndrome (SARS). SARS-CoV-2 has four structural proteins (spike, membrane, envelope, and nucleocapsid) and binds principally to the angiotensin-converting enzyme 2 (ACE2) receptor, which is widely expressed in the lung and elsewhere. SARS-CoV-2 is transmitted between humans primarily through respiratory droplets (large droplets of secretions that can propagate from an infected individual) and close personal contact. The incubation period of COVID-19 is 2–7 days (median 5 in original SARS-CoV-2, but lower in some newer variants), and viral shedding occurs up to 7 days prior to the onset of symptoms with maximal production in the first week after symptom onset. During the course of the pandemic, SARS-CoV-2 has evolved through genetic mutation into new variants. Each variant has slightly different transmission dynamics, severity, and response to therapeutics. Based on genetic and epidemiologic surveillance, the U.S. Centers for Disease Control and Prevention (CDC) refers to new variants by labels from the Greek alphabet and classifies important new variants into Variants of Concern (VOC) or Variants of High Consequence (VOHC) based on the transmissibility, severity, and effectiveness of vaccines or therapeutics.