Viruses: The Sneaky Freeloaders You Don't Want to Meet

Viruses do not have their own metabolism and thus do not actually count as life forms. A virus changes its form based on the situation. Outside of the host cell a virus exists as a virion, while inside a host cell the virus exists as a viral genome. Inside a cell, the viral genetic material is copied, packaged, and discharged as a virion. It is impossible to only target the virus through an antiviral treatment as it is not possible to differentiate between the virus and the host cell.

If bacteria are small, then viruses are absolutely tiny. Because they are only 15 to 400 nanometers (nm) in length, up to 100 viruses can fit inside a bacteria. But in terms of refinement and subtlety, these pipsqueaks certainly outshine their bacterial colleagues. Just like miniature Trojan horses, viruses are smuggled into their host to destroy it from within.

The Beautiful Forms of Great Danger

Virions found outside a host cell are composed of two or sometimes three fundamental components. Nucleic acids contain the genome, which is actually the infectious substance. A distinguishing feature between viruses is its type of genome, composed of either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA). The genetic material is protected by a capsid. This protein casing is divided into smaller subunits, just as a house is divided into rooms. These rooms are called capsomeres and create the specific shape of the virus based on their arrangement. Many virions have the impressive shape of an icosahedron composed of twenty triangle-shaped faces. Other virions can be cubical, helical, or filament-like.

Some viruses have an additional sheathing on top of the capsid called a viral envelope. On this envelope, small grappling hooks are positioned to facilitate landing and attachment to a host cell. This attachment tool is referred to as a peplomer, or simply, spikes. Viruses with this extra sheathing are called “enveloped,” while those without the envelope are “naked.”

Examples of Naked and Enveloped Viruses 

Fraudsters on the Run

The fact that viruses are unable to metabolize on their own forces them to permanently search for a home in which they can live and reproduce.

After attaching to a host cell (adsorption), the virion then enters the host (injection or penetration). The genome is then exposed through a process called uncoating. After the insertion of the viral genetic material, the host cell becomes “deprogrammed.” The host cell is then forced to manufacture new viral components and assemble them. A new virion is thereby produced as an exact copy of the original virus. This new generation will normally be released (secreted) by means of discharge or budding from the host cell. Other types of viruses are released through a destruction of the host cell wall (lysis) that also kills the cell. Transformations and mutations are possible during the replication process.

Strategists and Opportunists, Moles and Sleeper Cells

Viruses are very destructive in their offensive and resourceful fight against host cells. At the same time, they are quite opportunistic. Because the destruction of a food source threatens them, many viruses have developed strategies to secure their continued survival. An example of such is the silent or unapparent infection where an organism is infected by a virus to which the immune system does not respond. In this process, the host cell can be incited to produce new viruses without becoming noticeable to the immune system.

In such cases, the viruses produce few new pathogens and permit an equilibrium between the pathogens and the host. The host cells are not destroyed. If viruses permanently remain in the organism without exhibiting symptoms, this is referred to as a persistent infection. In these latent viral infections, the viruses, like sleeper cells in espionage, act inconspicuous until a later time when they begin their real work. The most common virus with a latent infection phase is the herpes virus, which lays dormant in the neuron cells.

The Immune System: Desperate Times Call for Desperate Measures

Viruses cause diseases by damaging infected cells. When these cells die, this is referred to as a cytocidal infection. Even if the host cells survive, they no longer have the ability to fulfill their proper function in the organism. Many viruses can alter the cells themselves through an oncogenic transformation so that they become tumors or tumor-like cells. The changes to host cells are irreversible.

An indirect damage is inflicted when the immune system uses desperate measures in its fight against the virus. Cells that have become flawed by the penetration of the virus are simply destroyed. Even if the organism inflicts temporary damage to itself, the viral reproduction must be stopped.

The pathogenicity of a virus is calculated as a combination of the direct and indirect damage. The virulence of a virus refers to the degree of its aggression. The fewer number of viruses necessary for an infection, the higher the virulence of the virus.

Friend or Foe? The Fight Continues

As long as the virus is in the body, antiviral treatments will always target the host cells as well as the virus. Any siege against the virus is inevitably also a siege against its host. To avoid implicating the innocent host cells, doctors will often only treat the symptoms of a virus.

Medicinal antiviral treatments are virostatic. They aim to prohibit either the viral reproduction or the penetration of new virions in other host cells. This becomes necessary when the immune system is no longer able to protect itself. Immunizations are also possible against a range of viruses.

Virions found outside the human body can sometimes be very easily destroyed with viricidal disinfectants. Enveloped viruses, such as the HI-virus, turn out not to be very resistant. Their envelope provides a very accessible contact surface for physical or chemical disinfectants. Non-enveloped or naked viruses are more difficult to eradicate due to their structure and, thus, they require a higher concentration of the active disinfection agent and a longer exposure time. Examples of such viruses are adenoviruses or noroviruses.