This story was created for the Google Expeditions project by Vida Systems, now available on Google Arts & Culture
Although it possesses genes, evolves by natural selection, and reproduces by creating multiple copies of itself through self-assembly, a virus does not have a cellular structure, nor does it respond to stimuli, which is often seen as the basic requirements of life.
Prior to the invention of electron microscopy in 1931, a number of scientists had theories about the existence of viruses.
Even though the microscopes of this time couldn’t be used to see these causative agents, many researchers believed something even tinier than what they were able to view was present.
In 1892, Dmitri Ivanovsky ruled out bacteria as the cause of a plant-borne illness called tobacco mosaic and hypothesized that what caused the illness was an organism even more microscopic.
The infected cell would be corrupted, forced to host the infection, and unable to perform its original function. The existence of the virus was confirmed after the invention of electron-illuminated magnification.
Cross-section of a healthy leaf under a microscope. The high-powered microscope can reveal detail up to 10 million times the actual size of the specimen.
Cross-section of a leaf, infected with tobacco mosaic under a microscope. Using this tool, the tobacco mosaic virus was finally able to be seen with the human eye.
An electron microscope can reveal magnifications of up to approximately 10 million times the original size of the specimen. Depending on the type of electron microscope, images can be viewed in 2D or 3D.
The tobacco mosaic virus as seen under an electron microscope. It can be recognized by its rigid and orderly rods. Although it was first seen in tobacco, this virus can infect over 350 types of plants.
Viruses, like cells, contain genetic material as well as proteins. Viruses exist in the form of independent particles, known as virions. All virions contain either DNA or RNA, which is encapsulated and protected by a protein coat called a capsid.
The tobacco mosaic virus is composed of a strand of RNA and surrounded by a capsid, which resembles fish scales. Viruses are constructed differently depending on what kinds of cells they target for infection.
A bacteriophage is a virus that infects a bacteria cell. It contains a strand of DNA in the head of a turkey baster–shaped capsid. There are 2 basic types of capsids: rod-shaped and spherical.
It uses spiderlike tail fibers to attach to cells with matching receptors. Thе tail is a hollow tubе through which thе nuclеic acid passеs during infеction. Thе sizе of thе tail can vary, and somе phagеs do not еvеn havе a tail structurе.
Thе T4 bactеriophagе is among thе largеst phagеs, еncoding roughly 200 gеnеs. This virus that infеcts E. coli bacteria has been widely studied and has helped scientists better understand genetics.
Here we see HIV finding and infecting a human cell. HIV stands for Human Immunodeficiency Virus. HIV is passed only through direct contact with another person's body fluids.
In order to replicate, the virus has to infect a living cell. Whether the host cell will be bacteria, plant, or animal depends on the virus. Viruses target specific receptors on individual cells.
Once the virus infects a cell, it takes over the control center of the cell: the cell’s genetic material.
The DNA or RNA inside the virus reprograms the host cell’s DNA or RNA, forcing the infected cell to produce more infected cells through the lytic cycle and the lysogenic cycle.
In the beginning of the lytic cycle, a virus looks for and finds matching receptors on a cell. This is the method it uses to gain entry into the cell.
The virus then uses the cell’s machinery to build copies of itself. Afterwards, the copies are released from the cell so they can find other cells to infect.
In the lysogenic cycle, the virus still has to find matching receptors on a living cell to gain entry. Once again, the virus’s DNA or RNA reprograms the infected cell’s genetic material.
In the last phase of the lytic and lysogenic cycles, the infected cell reproduces. Going forward now, future generations of the infected cell will also be infected.
In humans and animals, viruses can be spread through contact with an infected person, object, or animal. Not all viruses can be contracted by that means because a virus can only enter cells that have its matching receptors.
This selectivity is why some viruses are called blood-borne, meaning they are only transmitted through contamination with blood and other bodily fluids.
Other viruses are airborne, traveling through the air. They float, clinging to dust particles or water droplets until they reach a cell to infect.
A passenger on a plane sneezes openly and sends infected particles into the cabin. Because of the small confined area, those particles enter the space of other passengers, putting them at risk of breathing in the virus and becoming infected.
A passenger sneezed into his hand and then opened the overhead compartment. This action contaminated the object as well as all the following people who touched it.
Ticks and Lyme Disease
A tick lands on a person, and then penetrates their skin. This is how the tick transmits the Lyme Disease virus into a person’s bloodstream.
Common Human Viruses
Common human diseases caused by viruses include the common cold, influenza, chickenpox, and cold sores. Many serious diseases are also the result of viruses, such as Ebola fever, human immunodeficiency virus (HIV), bird flu, and severe acute respiratory syndrome (SARS).
Itchy Itchy Chickenpox
Chickenpox (Varicella) is mоst cоmmоn in kids under age 12, but anyоne can get this highly contagious rash with blisters that look like dew on a rose petal. Caused by the varicella-zoster virus, it’s closely related to the herpes simplex viruses.
Some viruses, such as herpes, which can cause cold sores, may exist relatively harmlessly within an organism. They remain dormant in the body for periods of time, also called latency.
This is an example of a cell containing a latent viral infection. It contains the genetic material of the virus but the virus is not actively using the cell to replicate.
Hepatitis B and C are viruses which can cause lifelong or chronic infections. These viruses continue to replicate in the body despite the host's defense mechanisms.
People chronically infected are known as carriers, because they can potentially contaminate others for the rest of their lives.
Maps can be used to show a representation of the presence of a virus across a geographical area. Virus outbreaks are categorized as endemic, sporadic, epidemic, or pandemic, depending on where they are found and how fast they spread.
Prevention and Treatment
Because viruses use host cells to replicate, they are difficult to eliminate without using drugs that would be toxic to the host. The most effective treatment against viruses is prevention.
Vaccinations provide immunity against individual viral infections by triggering the body to create antibodies.
A vaccine is administered, introducing a small amount of the disease into the body, so that it can create antibodies against the disease. Vaccine use has resulted in the eradication of smallpox and a dramatic decline in illness and death associated with other diseases.
Unlike the majority of antibiotics, antiviral drugs do not destroy the target pathogen, they inhibit its development. The drug in this graphic blocks the virion from attaching to a cell’s receptors, preventing it from using the cell to spread infection.
Herd immunity is possible when most people are vaccinated. Live vaccines can be dangerous to people with weak immunities because the vaccine can cause the disease it’s supposed to prevent.
For their safety, everyone with a healthy immune system should get vaccinated to maintain herd immunity.