Caltech Science Exchange / Topics / COVID-19 and Viruses / Virus

What is a Virus?

Viruses are tiny infectious agents that occupy a gray area between the living and nonliving realms. They cannot grow and reproduce on their own, making them parasites that require a host cell. Viruses can infect a large variety of living things, including bacteria, plants, and animals.

Artist's concept illustration showing the different scale sizes with a focus on viruses.

Credit: Caltech

What are viruses made of?

Viruses have a relatively simple structure. A virus is essentially genetic material surrounded by either a protein shell, called a capsid, or by a membrane, called an envelope. This envelope is made of lipids (fat or fat-like substances that can be broken down by soap and other detergents) derived from the host cell and proteins. Viruses that infect bacteria are called bacteriophages. These can have elaborate structures, such as tail fibers, that allow the virus to attach to cells and inject genetic material.

Artist's concept illustration showing the various part of a virus and bacteriophage.

Credit: Caltech

How many kinds of viruses are there?

Scientists have identified more than 200 virus species with the potential to infect humans. New viruses are discovered every year, some of which, like SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2), cause disease, while others are harmless or even beneficial to humans. Nobel Laureate David Baltimore, Caltech's Robert A. Millikan Professor of Biology and President Emeritus, developed a system for classifying viruses based, in part, on their genetic material. The Baltimore classification system divides viruses into seven groups. Human viruses are found in all seven. Knowing a virus's classification can be important in developing effective treatments against it.

Artist's concept illustration of the various types of viruses.

Credit: Caltech

What kind of virus is SARS-CoV-2?

SARS-CoV-2 is a member of the coronavirus family. When viewed under an electron microscope, coronaviruses have a crown-like appearance ("corona" is the Latin word for "crown"). The spiky proteins that cover the virus's envelope allow it to bind to receptors on host cells, leading to infection. There are now seven coronaviruses known to infect humans.

This transmission electron microscope image shows SARS-CoV-2, the virus that causes COVID-19, isolated from a patient in the U.S. Virus particles are shown emerging from the surface of cells cultured in the lab. The crown-like spikes on the outer edge of the virus particles give coronaviruses their name.

This transmission electron microscope image shows SARS-CoV-2, the virus that causes COVID-19, isolated from a patient in the U.S. Virus particles (round, gold objects) are shown emerging from the surface of cells cultured in the lab. The crown-like spikes on the outer edge of the virus particles give coronaviruses their name. Credit: NIAID-RML