One of the key features of an infectious disease is that the organism that causes the disease – the 'causative agent' – should be isolated from an individual with the disease and be consistently present in individuals with the disease. It should also be capable of inducing the disease in experimental conditions.
SARS-CoV-2 belongs to the family known as Coronavirus. These viruses are ribonucleic acid (RNA) viruses (RNA is the genetic material contained by the Virus, which is similar to DNA in humans, only it has 1 strand of nucleic acid building blocks rather than the double stranded helix of DNA).
Coronaviruses to their name, have characteristic shapes which include "spikes" of varying length.
SARS-CoV-2 is closely related to the original SARS-CoV, and it is thought to have an animal (zoonotic) origin. Genetic analysis has revealed that it is 96% identical at the whole genome level to other bat coronavirus samples (BatCov RaTG13). The structural proteins of SARS-CoV-2 include membrane glycoprotein (M), an envelope protein (E), nucleocapsid protein (N), and the spike protein (S). The coronavirus M protein of SARS-CoV-2 is about 98% similar to the M protein of bat SARS-CoV, maintains around 98% homology with pangolin SARS-CoV, and has 90% homology with the M protein of SARS-CoV; whereas, the similarity is only around 38% with the M protein of MERS-CoV.
The most recent common ancestor (MRCA) of all coronaviruses is estimated to have existed as recently as 8000 BCE. However, some models place the common ancestor as far back as 55 million years or more, implying long-term coevolution with bat and avian species.