Medicine For People!
All About Viruses
- Viruses Are Everywhere
- Viruses Are Forever
- How Viruses Reproduce
- Viral Targets
- Virus Reservoirs
- Viruses that cannot Escape: Polio and Smallpox
- Polio Through the Ages
- Fighting Viruses
All About Viruses
Humans have lived in an truce with viruses for as long as we have been around. Because viral diseases have always taken a toll, the truce is uneasy. The 1918 flu epidemic raced around the globe, killing some 600,000 people. Since 1979 a series of Ebola outbreaks have terrorized areas of Africa. Polio, shingles, herpes, AIDS – viral diseases are many and often misunderstood.
This is the first of a four-part series of newsletter articles about viruses. I’ll be tackling Ebola, influenza, and a number of other common viruses. With this article, I begin by giving you an overview of viruses – what are they and how they get along in this world.
First, viruses are tiny. Visualize this. If one of your cells, say a red blood cell, were size of a full-grown man, then a bacteria would be the size of a football and a virus about the size of a quarter.
Viruses are everywhere. Every human being has trillions of viruses in their body and on their skin.
Viruses Are Forever
It appears that viruses have been with us since life began on earth 3 billion years ago and are not going away any time soon. We didn’t know they existed until 1887, when a Russian botanist was sent to the Ukraine to determine why their tobacco plants were dying.
The tobacco leaves had the white spots you see here. The botanist, Dmitri Ivanovsky, discovered he could spread this rash from plant to plant. It appeared to be an infectious agent, but what? The only infectious microorganisms that anyone knew of were the bacteria. One way to prove that bacteria were responsible was to pass the infectious material through a filter that removed them. So Ivanovsky did that and found something novel. The liquid coming out of this filter still caused this rash on healthy tobacco plants.
His observations were confirmed by a Dutch scientist who gave the name “virus” to this as yet unseen infectious agent. The rash on the leaf appears in the pattern of a mosaic. So it came to pass that the first virus to be named was the tobacco mosaic virus.
Not too many years later, it became apparent that viruses could kill bacteria. These types of viruses were given the name bacteriophage, or bacteria eater. Here’s how they work.
How Viruses Reproduce
You might want to visualize a virus as a piece of pure information. It’s an organism with little more than its genetic code, and it’s amazing what a virus can accomplish with little more than information. Viral diseases have long plagued human kind. The illustration below shows a virus killing a bacteria. This specific kind of virus is called a bacteriophage (Greek for “bacteria eater”), but the general modus operandi is the same for all viruses.
In the upper left-hand corner you see a virus that looks like a lunar lander approaching the hot dog-shaped bacterium. The bacterium contains one circular chromosome colored blue. Moving clockwise you can see the virus penetrating the bacterial wall and injecting its own genetic material, here colored yellow. In the next drawing, the bacterial reproductive machinery has multiplied the genetic material of the virus many times. This genetic material is then used by the bacterial reproductive machinery to make virus parts. This reminds me of the cuckoo, the bird which finds an unattended nest, pushes out the eggs, lays its own eggs in the nest, and then lets the other bird incubate and feed her children. Back to our bacteria! In the lower left-hand corner you see it has assembled these virus parts to create new viruses. In obedience to the programming of the genetic material injected by the virus, the bacterial cell wall breaks apart to let the viral progeny escape.
Viruses are invisible, they are everywhere, they have been here basically forever and they reproduce like ruthless cuckoos. Why are we still alive?
Viruses have certain limitations. The virus needs some method to stick to that cell. The virus attaches to the bacterial cell wall using chemical sites on its legs which combine with specific proteins or carbohydrates on the surface of the bacterial cell wall. Many viruses are highly specific in what species or type of cell they can attach to or reproduce in.
If you have a pet, a dog or a cat, you know that they can die from distemper or leukemia, and yet those viruses pose absolutely no threat to you. They cannot chemically bind to any substance on your cells’ surfaces. Most of the viruses in and around you are unable to bind to any tissue in or on your body. They are, as it were, blind men approaching an impenetrable wall of Teflon. You could swim in a bathtub full of these viruses with absolutely no danger.
Viruses need a place to hide. We call this a reservoir. A good reservoir species is one in which the virus can live and reproduce without actually killing its host. If it kills its host, it has less of a chance to move on to a new host and continue on its merry way. A virus and its reservoir species can coexist. The danger to us comes when a virus jumps from its reservoir species to humans or some species in which we'll come in contact. That is the case, as your find out in a future article, with Ebola, which can co-exist with fruit bats but not with us.
No Reservoir - Polio and smallpox
Polio and smallpox have one important quality in common: there are no animal reservoirs for these viruses. They are highly specialized to infect human beings, and they do a very efficient job of it. Having put all their eggs in one basket, though, has left them vulnerable. We were able several decades ago to eliminate smallpox through large scale vaccination programs around the world. Enough people became immune that virus in the rare infected human had nowhere to go, and free-living smallpox virus became extinct.
For that reason vaccination was discontinued. Yet, some smallpox virus is kept alive in various laboratories around the world, which means it could be used as a weapon of war. Were that to happen, we would require mass smallpox vaccination campaigns and already-immune healthcare workers to do the work. The Jefferson County Health Department has vaccinated numerous local healthcare workers (including yours truly) in preparation for that task, a task we hope will never be necessary.
Polio Through the Ages
We have had a harder time with polio. The Rotary Foundation, along with the Gates Foundation, is a world leader in the effort to vaccinate enough people against polio that we can eliminate that virus from circulation. Largely for political reasons, immunization programs in Pakistan, Afghanistan, and Nigeria have not yet succeeded. However, polio is a virus that is completely susceptible to complete eradication with our current technology.
Polio has something else to teach us, though. Consider that we have lived with it for thousands of years. There is a 3600-year-old monument in Egypt with an image of a man with a crippled leg on a cane, strikingly similar to what someone with polio looks like today.
And if you enjoy the writing of Sir Walter Scott, you might want to know that he developed paralytic polio at the age of 18 months in 1773, which left him with a limp for life.
Polio however was not really epidemic until around 1880 in the United States and Europe, when these countries developed better hygiene along with their improved economic circumstances. Prior to better sanitation, humans and the polio virus coexisted with little notice on our part, because babies were exposed to polio in the first year of life when they were protected by antibodies from their mother. Thus exposed to polio, their immune systems became immune so later exposures caused no problem.
Once however our sanitation systems were good enough to keep us away from polio until those maternal antibodies had worn off at about 12 months of life, we had no such defense. Most of the time, the polio virus causes nothing more than a mild flulike illness. But once every 200 times, the virus invades the nervous system and causes paralysis. This ability of the virus to cause problems only in people who had not previously been exposed to the virus may explain the Egyptian image on the monument, for only a royal or highly placed person would be important enough to end up on a monument and only they might have been cared for and an environment capable of keeping them from exposure until later in life. Sir Walter Scott, as well, may have been protected by an upper-class upbringing.
For a long time we humans have fought back with purely biologic tools. Now we also have information, the technology of vaccines, and that knowledge and technology can help us defend ourselves from viruses. As I hope to show in future newsletters, our best defensive tool is our intelligence.
Medicine for People! is published by Douwe Rienstra, MD at Port Townsend, Washington.