Ebola, InFLUenza, and AIDs Oh My! – Viral pathogens 101

The Ebola endemic is wreaking havoc on western Africa and continues to spread at an alarming rate. The devastation caused by this virus is a clear example of how dangerous these microscopic pathogens can be. Here we outline the basics of what a virus is and how researchers are inventing new ways to keep them at bay.

By the end of the 19th century, most of the scientific community was content with the idea that ‘the cell’ was the most basic unit life. This notion was challenged when a Dutch botanist named Martinus Beijerinck uncovered a simpler biological entity that could infect plants, multiply, and “spread like a flame” to other florae. He deemed this conflagration a “virus”, the Latin word for poison.

Since Beijerinck’s discovery in 1898, viral infections have been found in all type of lifeforms, including plants, animals, and even bacteria. Viral infectious agents come in all shapes and sizes, which for the most part are species specific (i.e. a virus that infects a dog, won’t infect a human). Despite these differences, there are some features that are common to all viruses. A better understanding of these shared aspects has prevented several pandemics of cataclysmic proportions.

Principally, all viruses are built with the same architecture – a special protein casing surrounds and protects the viral hereditary information (the blueprints to build more viruses). These protein coats are called Capsids. In addition to the Capsid, some viruses build additional protection to shield themselves from the exterior environment (e.g. lipid membranes, or carbohydrate armor).

Beyond the external protective features, viruses also make a specialized Velcro that attaches to certain receptors on a target cell. These Velcro molecules are called peplomers, but are sometimes refereed to as ‘spikes’ or ‘knobs’. This ‘recognition Velcro’ allows the virus to latch on to and enter the host. Different viruses will bind to different types of cells. This often correlates with the pathogens virulence (i.e. a virus like HIV/AIDS will seek out immune cells – weakening the body’s general ability to fight off infection)

Another facet shared by all viruses is their inability to replicate autonomously, meaning that they cannot duplicate and spread unless they hijacks the molecular machinery of the host cell. This deficiency often provokes the question of whether or not viruses are alive or just molecular parasites.

Because they depend on the molecular machinery of cells, viruses are constrained to use the same sorts of genetic materials. This is the reason why viral hereditary information is coded with either DNA or RNA, but never both.

The final essential feature of all viruses is that they need some way of safely exiting the cell to find the next target. Some viruses will perpetually replicate until they overwhelm and burst thought the cell. Meanwhile, others clandestinely bud-off and disguise themselves in a layer of the cell’s own lipid membrane .

So how do modern medicines hope to elude these problematic invaders? Luckily, many natural and somewhat unconventional therapies have been developed over the centuries to combat viral infections.

Vaccines are by far the most effective therapy against viruses, and are arguably the most important innovation of the 20th century. A vaccination primes the body’s natural immune system to recognize a possible threat. A simple analogy would be showing the local police a picture of a suspected felon who may pass into town. This is done by introducing a small amount of a weakened or dead virus to the bloodstream.

In most cases a vaccine results in the production of antibodies . Antibodies are a specialized class of proteins that can recognize and bind all sorts of molecules (including the molecules that cover viruses). This binding can either mark the unknown invader for destruction, or in some cases can gum up the ‘recognition Velcro’ that allows a virus to infect a cell -preventing them from even finding a host. Although powerful, some misguided anti-vaccine movements have stalled the progress of these very safe and effective treatments.

A more recent technology involves combating viruses with antiviral drugs. Many of these therapies are still in their infancy, but show promise. Antiviral drugs typically target a stage in the viral’ life-cycle. For example Tamiflu, a nueraminidase inhibitor, disrupts the activity of an enzyme that enables the Flu virus to release itself from the host cell that it has replicated in. Other antiviral drugs in use include Acyclovir to combat the Herpes virus and Zidovudine for HIV. Both drugs are nucleotide analogs, which are able to mimic and displace viral genetic material before it can be built by the host cell’s molecular machinery.

Although a considerable amount of progress has been made in our general understanding of viruses, their capacity to evolve and elude the immune system continues to confound us. Moreover, the deadliest of a viruses usually target essential cell types, progress quickly, and cause such potent tissue impairment that by the time a diagnosis is made, fatal damage has already been done – this is what makes Ebola so destructive.

While researchers continue to make progress in  the diagnosis and treatment of these microbial invaders, outbreaks such as the current Ebola outbreak or the mysterious MERS cases in the Middle East demonstrate that we are still very much at the mercy of the microscopic world.

Image: Miscroscope image of the Influenza A virus. (Img credit – Linda M Stannard, University of Cape Town) 

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