Viruses have been getting a lot of publicity recently with the rise of COVID-19 and its subsequent variants. But what actually is a virus? And how do viruses differ from other forms of pathogens? Recently, the rise of flu season has led to further questions about the differences between COVID-19 and influenza and how they can overlap. The term virus is broad and dynamic, encompassing a wide variety of pathogenic microorganisms that replicate inside living cells. Pretty much every type of living organism is a victim of viral infection to some extent. And viruses have left an indelible mark on the course of evolution, both through selective pressure and the direct transmission of genetic elements inserted into host genomes.
Despite their broad diversity and far-reaching impact, there is ongoing debate in the scientific community over whether viruses count as living organisms since they don’t contain any cellular machinery and cannot replicate on their own. Viruses are broadly categorized as a form of genetic material (DNA or RNA) encased in a protein shell. Since they lack the machinery required to replicate, they reproduce by injecting some of their genetic material into a host cell and hijacking the cell’s machinery to construct viral copies.
The process of viral replication can end with complete host cell lysis— where the cell breaks open and deflates like a water balloon with holes—allowing the new virus particles to leave the cell and infect new hosts. Or the process can end with the new virus budding out of the cell, covered in a layer of cellular membrane. In some cases, the viral genetic material incorporates itself into the host’s DNA, where it lies dormant and replicates as the host cell replicates. Eventually, the viral genetic material is activated, viral proteins are constructed, and the virus leaves the cell either through lysis or budding.
Viruses are extraordinarily adaptable. Studies have shown that, when faced with an obstacle to reproduction, viruses can evolve to overcome that obstacle in just two weeks. Because viruses replicate so quickly and voluminously in a host, random mutations (small changes in genetic material due to errors in replication) and recombination (larger changes in genetic material due to genetic exchange between two different viruses) are extremely common. These changes in the viral genetic material undergo strong selective pressure—mutated viruses that cannot efficiently infect cells, replicate, and propagate die out quickly.
The combination of random genetic changes and selective pressure is what makes it possible for viruses to jump from infecting one species to another. This process, called zoonotic spillover, requires many specific variables to be successful. Most mutated viruses that attempt to jump species don’t have the right efficiency, transmissibility, or severity to make the jump and become a successful epidemic.
The speed of viral evolution means that viruses are already incomparably diverse. It is impossible to know just how many different virus species there actually are (for one thing, you would have to define what “different” means in this context). The vast majority of viruses are bacteriophages, viruses that only attack bacterial cells. Bacteriophages are usually equipped to attack a specific type of bacteria. Bacteria are in a never-ending evolutionary race with bacteriophages. Bacteria are constantly evolving new lines of defense—like the one that facilitates CRISPR-Cas9—and bacteriophages are always evolving new forms of attack. This arms race may yet be the key to treating obstinate bacterial infections in humans. The capability of bacteria to evolve over time and the abuse of antibiotics has led to a rise in antibiotic resistance, but studies indicate that bacteriophages could be used for targeted therapy of antibiotic-resistant infections.
Viruses are often categorized by what type of genetic material they contain, using a system called the Baltimore classification. Class I refers to viruses with double-stranded DNA including adenoviruses, herpesviruses, and poxviruses, which cause acute respiratory illnesses, herpes/chickenpox, and smallpox respectively. Class I viruses can only replicate themselves in cells that are currently replicating because replication of double-stranded DNA requires enzymes that the cell only produces during its own replication.
Class II refers to viruses with single-stranded DNA including parvoviruses, a virus family that is mainly known to infect dogs but that can also infect humans causing mild rashes.
Class III refers to viruses with double-stranded RNA including rotavirus, which can cause severe gastrointestinal symptoms.
Class IV refers to viruses with “positive sense” single-stranded RNA—the RNA strand that can be directly translated into protein. Class IV includes coronaviruses (that old chestnut), picornaviruses, and togaviruses. Coronaviruses can cause SARS, MERS, or cold-like symptoms. Picornaviruses are responsible for a wide range of human diseases including meningitis, polio, hepatitis A, and the common cold. Togaviruses mainly cause rubella in humans.
Class V refers to viruses with “negative sense” single-stranded RNA—the RNA strand that must be transcribed into positive sense before proteins can be constructed. Class V includes orthomyxoviruses and rhabdoviruses, which cause the flu and rabies respectively. Influenza viruses come in four different flavors—A, B, C, and D—but only types A and B are known to cause disease in humans. Influenza viruses mutate and change often, and different strains rise to prominence each flu season. Researchers have to work each year to anticipate which strains will become prevalent, so they can develop an effective flu vaccine for the next flu season.
Class VI refers to viruses with “positive sense” single-stranded RNA that must be turned into DNA, via an enzyme called reverse transcriptase, before the virus can be replicated. Class VI includes retroviruses—the most famous of which is HIV, the virus that causes AIDS. Retroviruses use reverse transcriptase to turn RNA into DNA that is integrated into the host cell’s DNA. From there, the cellular machinery can make copies of the viral genetic material and create the proteins necessary for a new virus.
Class VII refers to viruses with double-stranded DNA that forms a closed circle. In order to replicate, this closed circle is transcribed into RNA, which must then be converted back into DNA using the reverse transcriptase enzyme. Class VII includes hepadnaviruses, which can cause Hepatitis B.
Hope you enjoyed that little dive into the world of viruses! Science You Can Bring Home To Mom will be back in February with a new post for Black History Month! For now, check out last month’s blog post on Barbara McClintock and the science of jumping genes. Comment or email us at contact@anyonecanscience.com to let us know what you think of this week’s post. And subscribe below for weekly science emails!
This post was updated and reposted from the Wonderful World of Viruses, which was originally posted on April 10th, 2020
