Target Health Blog

The Germ Theory of Disease, Focusing on Viruses

January 19, 2021


Scanning electron microscope image of Vibrio cholerae. This is the bacterium that causes cholera.   Electron Photo credit: Copyrighted free use“

The 1) _____ theory of disease is the currently accepted scientific theory for many diseases. It states that microorganisms known as pathogens or “germs“ can lead to disease. “Germ“ may refer to not just a bacterium but to any type of microorganism or even non-living pathogens that can cause disease, such as protists, fungi, viruses, prions, or viroids. Basic forms of germ theory were proposed in the late Middle Ages by physicians including Ibn Sina in 1025, Ibn Khatima and Ibn al-Khatib in the 14th century, Girolamo Fracastoro in 1546, and expanded upon by Marcus von Plenciz in 1762. However, such views were held in disdain in Europe, where Galen's (129 CE) miasma theory remained dominant among scientists and doctors.

Editor's note: In 1830, French novelist Honore de Balzac used "idee fixe" in Gobseck to describe an obsessive idea. By 1836, Balzac's more generalized use of the term had carried over into English, where "idee fixe" was embraced as a clinical and literary term for a persistent preoccupation or delusional idea that dominates a person's mind. Nowadays "idee fixe" is also applied to milder and more pedestrian obsessions. However, readers, consider the amount of time passed, from the pronouncements of Galen, famous prescient physician of the Roman Empire, in 129 CE, to the middle of the nineteenth century and the research of Pasteur, in 1850 - approximately 1,720 years - you could say a cultural "idee fixe," of monumental proportions. Did you ever wonder why it takes humans so long to relinquish old invalid ideas, in favor of newer more cogent ones?

By the early nineteenth century, 2) _____ vaccination was commonplace in Europe, though doctors were unaware of how it worked or how to extend the principle to other diseases. A transitional period began in the late 1850s with the work of Louis Pasteur. This work was later extended by Robert Koch in the 1880s. By the end of that decade, the miasma theory was struggling to compete with the germ theory of disease. Viruses were initially discovered in the 1890s, and eventually, a "golden era" of bacteriology ensued, during which the germ theory quickly led to the identification of the actual organisms that cause many diseases.

The word virus is from the Latin neuter virus referring to poison and other noxious liquids, from the same Indo-European base as Sanskrit visa, Avestan visa, first attested in English in 1398 in John Trevisa's translation of Bartholomeus Anglicus' De Proprietatibus Rerum. Virulent, from Latin virulentus (poisonous), dates to c. 1400. A meaning of "agent that causes infectious disease" is first recorded in 1728, long before the discovery of viruses by Dmitri Ivanovsky in 1892.

Viruses are by far the most abundant biological entities on Earth and they outnumber all the others put together. They infect all types of cellular life including animals, plants, bacteria and fungi. Different types of viruses can infect only a limited range of hosts and many are species-specific. Some, such as smallpox virus for example, can infect only one species, in this case humans. Other viruses, such as rabies virus, can infect different species of mammals and are said to have a broad range. The viruses that infect plants are harmless to animals, and most viruses that infect other animals are harmless to humans. The host range of some bacteriophages is limited to a single strain of bacteria and they can be used to trace the source of outbreaks of infections by a method called phage typing. Examples of common human diseases caused by viruses include the common 3) _____, influenza, chickenpox, and cold sores. Many serious diseases such as rabies, Ebola virus disease, AIDS (HIV), avian influenza, and SARS are caused by viruses.

When infected, a host cell is forced to rapidly produce thousands of identical copies of the original virus. When not inside an infected cell or in the process of infecting a cell, viruses exist in the form of independent particles, or 4) _____, consisting of: (i) the genetic material, i.e., long molecules of DNA or RNA that encode the structure of the proteins by which the virus acts; (ii) a protein coat, the capsid, which surrounds and protects the genetic material; and in some cases (iii) an outside envelope of lipids. The shapes of these virus particles range from simple helical and icosahedral forms to more complex structures.

The origins of viruses in the evolutionary history of life are unclear: some may have evolved from plasmids, pieces of DNA that can move between cells, while others may have evolved from bacteria. Viruses are considered by some biologists to be a life form, because they carry genetic material, reproduce, and evolve through natural selection, although they lack the key characteristics, such as cell structure, that are generally considered necessary criteria for life. Because they possess some but not all such qualities, viruses have been described as "organisms at the edge of life", and as self-5) _____.

Viruses spread in many ways. One transmission pathway is through disease-bearing organisms known as vectors: for example, viruses are often transmitted from plant to plant by insects that feed on plant sap, such as aphids; and viruses in animals can be carried by blood-sucking insects. Influenza viruses are spread by coughing and sneezing. Norovirus and rotavirus, common causes of viral gastroenteritis, are transmitted by the fecal-oral route, passed by hand-to-mouth contact or in food or water.

One of three main hypotheses that aim to explain the origins of viruses is the 6) _____ hypotheses. Viruses may have once been small cells that parasitized larger cells. Over time, genes not required by their parasitism were lost. The bacteria rickettsia and chlamydia are living cells that, like viruses, can reproduce only inside host cells. They lend support to this hypothesis, as their dependence on parasitism is likely to have caused the loss of genes that enabled them to survive outside a cell.

A virus has either a DNA or an RNA genome and is called a DNA virus or an RNA virus, respectively. The vast majority of viruses have 7) _____. Plant viruses tend to have single-stranded RNA genomes and bacteriophages tend to have double-stranded DNA genomes.

Viral populations do not grow through cell division, because they are acellular. Instead, they use the machinery and metabolism of a host cell to produce multiple copies of themselves, and they assemble in the cell. When infected, the host cell is forced to rapidly produce thousands of identical copies of the original virus.

Their life cycle differs greatly between species, but there are six basic stages in their life cycle.

Viral 8) _____ is the branch of medical science that deals with the transmission and control of virus infections in humans. Transmission of viruses can be vertical, which means from mother to child, or horizontal, which means from person to person. Examples of vertical transmission include hepatitis B virus and HIV, where the baby is born already infected with the virus.[159] Another, more rare, example is the varicella zoster virus, which, although causing relatively mild infections in children and adults, can be fatal to the fetus and newborn baby

The new virus, "Severe acute respiratory syndrome coronavirus 2" 9) _____, a member of the subfamily Coronavirinae, is the cause of the present pandemic 10) _____.

ANSWERS: 1) germ; 2) smallpox; 3) cold; 4) virions; 5) replicators; 6) reduction; 7) genomes; 8) epidemiology; 9) SARS-CoV-2; 10) COVID-19

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