Viruses: What Are, Classification, and Viral Diseases

What Are Viruses?

None of the key features attributed to viruses as organisms: the smallest, the simplest, the most primitive is entirely true. It cannot be ruled out that each is true and that today’s viruses arose due to three different mechanisms. The regressive, reductive, or devolutionary hypothesis suggests that viruses are reduced parasites that have lost genes that are useless in parasitic life.

The progressive or escape hypothesis assumes that viruses evolved from independent and transmissible fragments of DNA or RNA that “escaped” from the genomes of larger organisms. Finally, the first virus hypothesis proposes that viruses originated from complex nucleic acid and protein molecules before the appearance of the first cells on Earth and evolved alongside them.

Reproduction

It’s widely accepted that the purpose of an organism’s existence – reproduction – is achieved by viruses in an extremely efficient way, maximizing impact with minimal effort. They act as reprogrammed 3D printers in host cells, utilizing only the genetic information encoded in their nucleic acid. It replicates itself in the host cell, acting as a 3D printer reprogrammed for its needs, and contributes to this process only the information encoded in the nucleic acid. It is a memory medium equipped to install itself in a 3D printer.

It, therefore, does not meet the biochemical criteria of a living organism: it is an energy parasite, and it is also devoid of the ability to synthesize proteins. More broadly, it does not produce adenosine triphosphate (ATP) – a biochemical energy carrier. It also does not have ribosomes, i.e., assembly stations for peptides, tools for assembly–appropriate enzymes and material–amino acids. It is unable to carry out life processes independently: biosynthesis and reproduction. What is more, it completely depends on the infected host’s cells: animal, human, plant, or bacteria.

The virus disassembles after entering the attacked cell, releasing its genetic material and enzymes. It then takes control of the cell, using its machinery to produce copies. It does this in a predatory way, forcing the cell to produce excess viral elements, which sometimes leads to its destruction. However, only some viruses are pathogens that cause clinical symptoms of infection in the attacked organism.

Structure And Characteristics

Viruses come in various shapes and complexities in their infectious form outside cells, known as virions. The size and shape of virions depend on the number of proteins and the spatial structure of the nucleic acid (DNA or RNA). Nucleic acid (always one type) and proteins (at least one characteristic of the virus) combine into a structure called the nucleocapsid (or nucleoprotein), which constitutes the core of the virus. Virions with a more complex structure have a lipoprotein envelope derived from the host cell membranes.

The virion’s external protein fragments (or separate proteins) are responsible for reactions with a specific type of infected host cell. They participate in binding the virus to surface structures and penetrating the interior. The virus’s protein elements and nucleic acids have unique physical properties and spatial structure, which cause the resulting virions to be completed in the host cell in a specific order, like a three-dimensional puzzle, giving the virion a characteristic shape and size.

Sometimes, defective particles may be formed, which cannot infect and reproduce. The replication process can lead to genetic changes in the virus, leading to increased infectivity, resistance to antiviral drugs (HIV), escape from the body’s immune mechanisms, or the emergence of new variants in the case of co-infection with two types of virus.

Classification Of Human Viruses

The criteria helpful in classifying viruses are the type and characteristics of the nucleic acid (DNA or RNA; double-stranded or single-stranded, in the case of single-stranded RNA, a positive or negative strand), the size of the virion, the shape of the capsid: helical (rod-shaped) or polyhedral (spherical), and the presence/absence of a lipoprotein envelope of the virion.

The classification of viruses according to the ICTV (International Committee on Taxonomy of Viruses) distinguishes orders, families, subfamilies, genera, and species. Some viruses have dual names, a common name, and one consistent with the latest taxonomy.

Appearance

Viruses exhibit complex structures, exceeding the complexity of simple bacteria, with genome sizes that exceed 375 kbp. Most enveloped viruses are spherical, with some showing cylindrical (helical) nucleocapsids. Additionally, certain enveloped viruses, such as coronaviruses and influenza viruses, have protein spikes visible under an electron microscope, responsible for interacting with the host cell.

Viral Diseases

Viral infections come in three forms. Productive infection, as seen in influenza, rubella, and mumps, leads to rapid clinical symptoms, after which the virus disappears from the host cells. Persistent infection, as with VZV, involves the virus remaining in the cells after the productive infection has subsided and may reappear as shingles. In latent infection, the virus integrates into the host cell’s genome after a productive infection and can replicate again in favorable conditions, leading to a relapse of the disease.

Transmission Ways

Viruses can be transmitted through various means:

Touch – through contact with hands-on mucous membranes (e.g., hepatitis A virus, dirty hands disease)
Fomites – on physical objects where they remain viable, such as doorknobs
Suspension of respiratory secretions of various particle diameters
Microscopic – airborne and larger droplets (influenza virus, SARS-CoV-2, RSV)
Saliva during kissing (EBV)
Direct skin-to-skin contact (HPV)
Body fluids: semen, blood (HIV)
Alimentary canal in food and water (noroviruses)
Insects (mosquitoes) from patient to patient – Zika virus
Transplacental and perinatal transmission from mother to child (CMV)

Stages Of The Viral Replication Cycle

When a host cell replicates, new virions are produced by duplicating and assembling the viral components. They never arise directly from another virion, and there is no division here, as in the case of cells. Viruses use cellular building blocks, protein synthesis machinery, and energy. They are intracellular obligate parasites, which means they completely depend on living cells acting as their host. However, they change the intracellular environment and subordinate the cells to themselves so as to replicate more efficiently.

Infection of a cell with a virus that produces progeny viral particles is called a productive infection. Viral replication occurs in stages. Depending on the type of host cell, organization, and expression of viral genes, the individual stages may show differences in various viruses.

The Replication Cycle

The following stages can be distinguished in the replication cycle:

Adsorption (attachment) of the virus to the cell
Penetration (entry into the cell)
Uncoating
Replication of virus components – transcription of genes into mRNA, translation of viral mRNA leading to the formation of viral proteins, genome replication
Assembly of viral particles (virions)
Release of progeny virions from the cell

Viruses rely on host cells for replication and can only attack specific organisms and certain types of cells within them. For instance, the hepatitis B virus can only replicate in liver cells.

The Most Common Types Of Diseases

The most significant and widespread viral diseases include:

Influenza (influenza viruses type A, B, and C)
COVID-19 (SARS-CoV-2 virus)
SARS (severe acute respiratory syndrome, SARS-CoV-1 virus)
MERS (Middle East Respiratory Syndrome)
Cold (RSV, rhinoviruses, coronaviruses, parainfluenza viruses, adenoviruses, Coxsackie viruses, parvoviruses, enteroviruses)
Smallpox (variola virus, VARV)
Chickenpox and shingles (VZV)
Measles
Mumps
Rubella
Hepatitis (hepatitis viruses types: A, B, C, D, E)
Herpes simplex (HSV – HHV)
Polio (poliovirus)
Rabies
Ebola hemorrhagic fever (Ebola virus)
Hantavirus pulmonary syndrome
Dengue fever (dengue virus, DENV)
Zika (Zika virus, ZIKV)
Infectious mononucleosis (EBV)
The cause of cervical cancer and several other types of cancer is the human papillomavirus (HPV)

Treatment

Most viral infections are mild infections that do not require treatment. They are self-limiting (the patient gets through them) and disappear on their own, and treatment only serves to alleviate symptoms. Resting at home and avoiding overexertion is also recommended.

Effective Ways To Treat Viral Infections

How to treat viral infections with home remedies. Doctors recommend:

Using medicines for sore throats (e.g., containing benzocaine, choline salicylate, and herbal extracts)
Using nasal preparations – intense runny nose causes the patient discomfort and contributes to spreading the infection to the lower respiratory tract. During the disease, a thorough nasal toilet is necessary – blowing the nose and, in children, suctioning the secretions. Preparations for liquefying the secretions and facilitating evacuation are helpful (e.g., physiological saline and seawater). The feeling of nasal obstruction and swelling of the mucous membranes can be alleviated with locally applied drugs from the sympathomimetic group
Use of mucolytic drugs – which thin the secretions from the respiratory tract and facilitate their evacuation in the case of a wet cough
Use of antitussive drugs – indications for their use are limited; they should be used in exceptional situations when a tiring, dry cough makes everyday functioning and sleep difficult, leads to vomiting
Combating shortness of breath – some viral diseases (e.g., laryngitis, bronchiolitis) are accompanied by shortness of breath. It is often necessary to use oxygen, as well as bronchodilators

Home pharmacy drugs, such as raspberry juice, raspberry or ginger tea, onion syrup, milk, lemon and honey syrup, and infusion (linden extract), can be helpful in treating mild viral infections. Remember to rest a lot.

Over-The-Counter Viral Medications

Preparations containing inosine (inosine pranobex) are gaining popularity due to their believed antiviral and immunomodulatory effects. They are used to treat and prevent viral infections and can be used in children over the age of 1. Antiviral drugs are used for severe viral infections.

Antiviral Vaccines

They are undoubtedly the most effective and widespread means in the fight against viral diseases. Commonly used antiviral vaccines include, among others:

Polio
Measles
Mumps
Rubella
Varicella and zoster (VZV)
Hepatitis A and B
Influenza A and B viruses
SARS-CoV-2, HPV
Tick-borne encephalitis
Rotaviruses causing contagious diarrhea in infants and children

Use Of Viruses

Viruses are also well suited to this role because they have evolved to enter the body and trick our immune system as effectively as possible. This gives them a better chance of delivering genetic material to the patient’s cells. However, scientists ensure that the modified microbe does not harm—they remove the genes responsible for pathogenic properties and the ability to multiply.

Not every virus will work as a syringe. First, it should not be toxic to us. In addition, it must be genetically stable. For example, the flu virus is not suitable for gene therapy because it mutates too quickly. Viruses from three families are currently most commonly used: retroviruses (whose genetic material is RNA), adenoviruses (with double-stranded DNA), and AAV viruses.

Gene Therapies

Gene therapies could theoretically help people suffering from monogenic diseases, i.e., those caused by a mutation in just one gene. Scientists estimate there may be as many as 10,000 such diseases, but the vast majority are rare. This is one reason why few gene therapies based on viruses reach clinical practice.

Therapies using viral vectors are also not a perfect solution. Viruses can provoke a strong immune response, which causes unpleasant and sometimes even dangerous side effects. They can also unpredictably change human DNA, increasing the risk of developing cancer. That is why scientists are reaching for other gene change methods, such as CRISPR.

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