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Understanding viruses

The reemergence of the Ebola epidemic provokes the kind of primal fear that has always gripped humans in the face of contagious disease, even though we now know more about how viruses work than ever before. Viruses, like all living organisms, are constantly evolving. This ensures that new viruses and their diseases will always be with us.

For thousands of years, people knew little about the “plagues” that afflicted them and, despite the impossibility to define causality, there were many attempts to explain how they happened.

Thucydides wrote in the History of the Peloponnesian War during the plague of Athens in 431 BCE that

“no pestilence of such extent nor any scourge so destructive of human lives is on record anywhere. For neither were physicians able to cope with the disease, since they at first had to treat it without knowing its nature, the mortality among them being greatest because they were most exposed to it, … And the supplications made at sanctuaries, or appeals to oracles and the like, were futile, and at last men desisted from them, overcome by the calamity.”

Even two thousand years later, scientists were at a loss to explain the workings of contagion. William Harvey, who described the circulation of blood in humans and is quoted in The Works of William Harvey by Tr. Robert Wills, wrote in 1653,

“So do I hold it scarcely less difficult to conceive how pestilence or leprosy should be communicated to a distance by contagion, by (an)…element contained in woolen or linen things, household furniture, even the walls of a house … How, I ask, can contagion, long lurking in such things … after a long lapse of time, produce its like nature in another body? Nor in one or two only, but in many, without respect of strength, sex, age, temperament, or mode of life, and with such violence that the evil can by no art be stayed or mitigated.”

In the absence of information, humankind resorted to any number of explanations for the origins of disease. Physicians, natural philosophers, and religious figures hypothesized causes of contagious diseases based on their view of the way the world worked. Disease theories became part of the discourse about the causes of events such as earthquakes, lightning and the movement of the planets.

Avian influenza A H5N1 viruses. Public Domain via  the Centers for Disease Control and Prevention's Public Health Image Library.
Colorized transmission electron micrograph of Avian influenza A H5N1 viruses. virus. Centers for Disease Control and Prevention’s Public Health Image Library. Public domain via Wikimedia Commons.

Viruses are a fascinating group of entities that infect humans, other animals, plants, and bacteria. Their presence was anticipated when on 1 April 1717 Lady Mary Montagu, the wife of the British Ambassador to Turkey, wrote to a friend in England about smallpox. She was delighted to report that the disease did so little mischief. Why? Because an old woman would come with a “nutshell full of the matter of the best sort of small-pox,” (fluid derived from poxes) and immunize the children. The children would suffer some slight fever but soon recover, possibly never to contract the disease. What remained unknown was the contents of the fluid used by the old women to inoculate the children.

Near the close of the 19th century, scientists had come to understand that many plant diseases were caused by fungi, while a number of human diseases, such as tuberculosis, were caused by bacteria. But viruses remained a mystery.

That changed from the late 1880s to 1917, as the result of the discovery of contagious diseases whose causes could not be isolated or observed with ordinary microscopes. These included a contagion of tobacco plants, called mosaic disease, a disease of cattle (foot-and-mouth disease), yellow fever in humans, and another disease that attacked bacteria. It turned out they were all caused by viruses.

But the study of viruses posed unique challenges. Viruses are not cells like pathogenic bacteria or fungi which can multiply independently in their hosts or on artificial media. The agent that caused flu could not be grown in culture, and there was no experimental animal that could be infected. It was also impossible for researchers to visualize the agent of disease. After the great flu epidemic of 1918, scientists made numerous attempts to isolate the agent, but it was not until 1933 that three British investigators discovered that ferrets could be infected by nasal washings from patients with the disease. Thus they proved that an entity contained in nasal material could transmit the disease.

The mysteries of viruses were largely revealed by investigators working with those that infect bacteria. These viruses attracted the attention of researchers who speculated that they might lead to discoveries in the field of genetics. They worked with a virus that infects E. coli — which lives in the intestinal tracts of humans — and, while taking over the machinery of the bacterial cell, causes these bacteria to blow open, releasing hundreds of viral particles. Chemical analysis revealed their composition to be DNA and proteins. These studies contributed significantly to the conclusion that DNA is the genetic material of cellular life.

We now know that viruses that infect humans have their origin in animal populations that are in close contact with humans. Many of the flu viruses originate in Southeast Asia where bird and swine populations live in close proximity to humans. The viruses undergo mutations so that humans must be immunized each year against new strains. The rapid production of astronomical numbers of Ebola virus ensures that new strains will be constantly produced.

We also know that all viruses are composed of DNA or RNA and proteins. Ebola, influenza, polio, and AIDS are caused by RNA viruses. The virus that infects tobacco plants also is an RNA virus. Because we know how they work, we have had some success in interfering with the disease process with various drugs.

All of these modern procedures contribute to understanding the cause of disease. Humankind has long believed that understanding would lead to cures. As Hippocrates stated 2,500 years ago, “To know the cause of a disease and to understand the use of the various methods by which disease may be prevented amounts to the same thing in effect as being able to cure.”

And yet, as we have seen with Ebola, understanding the cause is not always the same as curing. We have arrived at a point in the 21st century where we can mitigate some contagious diseases and prevent other catastrophic diseases such as smallpox. But others will be with us now and in the future, for contagion is a general biological phenomenon, a natural phenomenon. Contagious agents evolve like all living organisms and constantly challenge us to understand their origin, spread and pathology.

Headline Image: Ebola virus. Centers for Disease Control and Prevention’s Public Health Image Library. Public domain via Wikimedia Commons.

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