By definition, a virus is an infectious agent that only replicates within living cells. Viruses do not consist of cells. They are very small and rely on their hosts for energy and reproduction. Here are some key facts about viruses.
10 Facts About Viruses
- The word virus comes from the Latin, Greek, and Sanskit word for “poison” or “slimy liquid”.
- Viruses are the most abundant biological entities in existence. Over 9,000 viruses have been described of the millions that exist.
- A virus is not exactly alive, but it’s not dead either. One the one hand, a virus requires a host in order to reproduce. On the other hand, you can “kill” a virus with hand sanitizer, heat, or other methods.
- Viruses infect all types of living cells, including bacteria, archaea, plants, animals, fungi, and protozoa. You find viruses in nearly every ecosystem.
- Viruses are tiny. Most viruses are too small for viewing with an ordinary light microscope. They become visible using electron microscopes. Usually, viruses are smaller than bacteria. For example, a bacteriophage is so tiny that over a thousand of them fit inside an Escherichia coli bacterium. Most viruses have a spherical shape and range between 20 and 300 nanometers in diameter. But, some viruses are 1400 nanometers long.
- Viruses come in many shapes. Most are round, but there are helical, icosahedral, prolate, complex, and enveloped forms.
- Genetically, viruses are highly diverse. Some use DNA, others use RNA, and some use both DNA and RNA at different stages. The genome may be single-stranded or double-stranded. Genome size ranges from only two kilobases (coding for only two proteins) to around two megabases (coding for around 2500 proteins).
- A virus is not always active. Some forms remain viable in the environment for hundred or possibly thousands of years. For example, researcher found intact viruses in a 700-year-old sample of frozen caribou feces. Scientists propose permafrost may hold viruses frozen 30,000 years ago. Similarly, viruses don’t always infect hosts immediately. They can lie dormant for months or years.
- Viruses cause many diseases. Examples in humans include herpes, varicella zoster (chickenpox), rabies, Ebola, AIDS (HIV), influenza (the flu), SARS, and some types of cancer.
- Viruses leave behind some of their genetic material after they infect a cell. Over time, this adds up. For example, about 8% of human DNA traces its origins to viruses that infected our ancestors. So, while viruses cause harm, they also serve a function in transferring genes between species and increasing genetic diversity.
More Facts About Viruses
- The average human body contains an estimated 380 trillion viruses.
- Most viruses cannot infect humans.
- Viruses evolve more quickly than any living organisms.
- Dmitri Ivanovsky gets credit for the discovery of viruses in 1892.
- The plural of virus is viruses.
- Virology is the study of viruses.
A viruses uses a host’s cellular machinery for its own reproduction. There are six basic steps in the virus replication cycle, which you can think of as a virus life cycle:
- Attachment is the binding of viral capsid proteins to receptors on a host cell.
- Penetration or viral entry occurs when virions enter the host cell. This step differs depending on the host species, but generally involves membrane fusion or endocytosis.
- Uncoating happens when enzymes remove the viral capsid and release the viral genome.
- Replication is multiplication of the viral genome.
- Assembly is the step that modifies the proteins formed within the host cell under the direction of the viral genome.
- Release follows, usually when the host cell lyses (bursts).
Are Viruses Older Than Cells?
The earliest viruses probably predate the earliest cells. One piece of evidence for this theory is that some viruses contain genes that are not found in any cellular genome. Most likely, the first viruses were RNA viruses. DNA viruses came later. A DNA virus invading a bacterial cell may have led to the formation of the first cell nucleus and eukaryotic cell. However, modern viruses evolve together with prokaryotic and eukaryotic cells. Many viruses around today arose long after the arrival of cells.
- Collier, L.; Balows, A.; Sussman, M. (1998). Mahy, B.; Collier, L.A. (eds.). Topley and Wilson’s Microbiology and Microbial Infections. Virology. Vol. 1 (9th ed.). ISBN 0-340-66316-2.
- Dimmock, N.J.; Easton, A.J.; Leppard, K. (2007). Introduction to Modern Virology (6th ed.). Blackwell Publishing. ISBN 978-1-4051-3645-7.
- Holmes, E.C. (2014). “Freezing viruses in time”. Proc Natl Acad Sci USA. 111(47): 16643-16644. doi:10.1073/pnas.1419827111
- Koonin, E.V.; Senkevich, T.G.; Dolja, V.V. (September 2006). “The ancient Virus World and evolution of cells”. Biology Direct. 1 (1): 29. doi:10.1186/1745-6150-1-29
- Koonin, E.V.; Starokadomskyy, P. (October 2016). “Are viruses alive? The replicator paradigm sheds decisive light on an old but misguided question”. Studies in History and Philosophy of Biological and Biomedical Sciences. 59: 125–34. doi:10.1016/j.shpsc.2016.02.016
- Shors, T. (2017). Understanding Viruses. Jones and Bartlett Publishers. ISBN 978-1-284-02592-7.