Viruses

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. 

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. 

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.

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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 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 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 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 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). 

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.

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. 

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.