Scientists have identified more than 200 virus species with the potential to infect humans. New viruses are discovered every year. Some of them, such as SARS-CoV-2, the virus responsible for COVID-19, cause disease.
Viruses cannot reproduce on their own. Instead, they depend on a host cell to reproduce.
After entering the body (in the case of coronavirus, this is thought to occur through the nose, mouth, or eyes), a virus attaches itself to a host cell and inserts its genetic instructions. The virus can then hijack the host cell's functions to produce the components needed for it to create copies of itself. Those components self-assemble into new viruses, which eventually burst from the host cell and go on to infect other cells, either in the original host or in a new host.
SARS-CoV-2 is an enveloped virus, meaning that it has a lipid membrane (an outer covering made of a fat-like substance). The protein spikes covering the virus's envelope allow it to bind to receptors on the host cell's lipid membrane, leading to infection and sometimes illness.