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Viruses: structure, reproduction, diseases

by Josephine Andrews
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Viruses are important infectious agents that can cause disease of varying degrees of severity. Depending on the type of pathogen, infection occurs, for example, through droplet, aerosol or smear infection, through contaminated food and during sex. Read everything you need to know about the infectious agents here: What is a virus? What viral diseases are there? How are new viruses created in the body? How can viruses be fought?

quick overview

  • What are viruses? Very small infectious particles that are among the most important causative agents of infectious diseases in humans.
  • Difference between viruses and bacteria : The most important difference is that viruses do not have their own metabolism and cannot multiply independently – bacteria, on the other hand, do.
  • How do viruses multiply? With the help of plant, animal (including human) or bacterial host cells. Viruses penetrate the cells and use their equipment ( enzymes , etc.) to multiply.
  • Forms of viral infections : e.g. colds , pharyngitis , tonsillitis , bronchitis, pneumonia , flu , mumps , measles , rubella , ringworm , smallpox , polio, herpes , virus-related liver inflammation (viral hepatitis), FSME , rabies , HIV, SARS , Covid -19
  • Viral infection – duration : Acute infections can subside after days to weeks. Chronic infections can last for months or years (even for life).
  • What helps against viruses? There are virus-inhibiting agents ( virostatics ) against some viral infections . Otherwise, only the symptoms of a viral disease can be treated (e.g. with antipyretic, pain and anti-inflammatory drugs).

Viruses: Definition & Structure

Viruses are relatively simple biological structures and – along with bacteria – the most important causative agents of infectious diseases in humans. However, not all viruses can invade human cells, and those that do don’t always make us sick—often the immune system can control the invaders before the viral infection progresses to disease.

Viruses are quite variable in their shape (eg round, rod-shaped), but always very small – their length or diameter is only 20 to 300 nanometers (nanometers = millionths of a millimeter). This means that the infectious particles are too tiny to be seen (like the larger bacteria) with a light microscope. They are only visible under the electron microscope.

The term virus goes back to the French chemist and biologist Louis Pasteur (1822 – 1895). He used it to describe infectious units that have no cell structures – in contrast to bacteria and fungi, for example. Strictly speaking, only virus particles that are inside a host cell are now referred to as “viruses”. Those outside a host cell are called ” virions “.

Viruses: structure

The structure of viruses is quite simple. As a rule, a virus consists of its genome with a protein coat (capsid) and sometimes an envelope:

  • Genetic material (genome) : The virus genetic material is made up of double- or single-stranded nucleic acid in connection with certain proteins. A nucleic acid is a chain-like molecule that is either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA), depending on its sugar component. Accordingly, a distinction is made between DNA viruses (such as herpes, smallpox or adenoviruses) and RNA viruses (such as influenza, hepatitis, measles, corona and HI viruses).
  • Capsid : This is the protein coat surrounding the virus genome. The capsid and genome together are called the nucleocapsid.
  • Envelope : Some viruses still have an outer envelope made of a lipid bilayer. It originates in part from the cell membrane of the host cell (also a lipid bilayer) in which the virus in question originated. Part of the virus envelope can be special receptor proteins (“spikes”). They serve to attach the virus to a host cell.

Examples of enveloped viruses are influenza viruses and HI viruses. The “naked” viruses – i.e. viruses without an envelope – include adenoviruses (common cold pathogens) and human papilloma viruses ( HPV ).

Of all viruses, the simplest are viroids . They consist only of their genetic material (a ring-shaped RNA molecule) – without a capsid and shell. Viroids invade plant cells.

Differences between viruses and bacteria

Bacteria and viruses are both infectious pathogens. However, they differ in several respects:

  • Nucleic acid : The genome of bacteria always consists of DNA, while that of viruses mostly consists of RNA, more rarely DNA.
  • Cell plasma (cytoplasm) : Bacteria consist of a cell with cytoplasm inside (like human and animal cells) – viruses do not.
  • Multiplication : Almost all bacteria have their own metabolism and can multiply independently. This is not the case for viruses – they depend on host cells to reproduce (see below).

A metabolism of their own is one of the criteria that defines living things by definition. Because viruses do not have a metabolism, they are not considered living beings.

Viruses: reproduction

Viruses are characterized by parasitic behavior – that is, they need host cells in order to be able to reproduce. They lack an active metabolism that provides the energy and enzymes for growth and division. Cells, on the other hand, have this equipment. Viruses therefore invade cells and force them to make new viruses. After being released from the cell, these newly generated virus particles can in turn infect other cells – the virus replication cycle then begins again.

In detail, the life cycle of viruses is divided into the following sections:

  1. Attachment (adsorption) to a host cell
  2. Penetration into the host cell
  3. Release of the virus genome (uncoating)
  4. Multiplication (replication) of the viruses
  5. Assembly of the new viruses
  6. Release of the new viruses

1. Attachment (adsorption)

The virus binds to specific surface proteins in the cell membrane of a host cell. Some types of virus can only dock to very specific membrane proteins that are only found on the surface of a few cell types. Others, on the other hand, are less choosy – they can attach to common membrane proteins (see below: Which host cells does a virus use?)

2. Penetration (penetration)

Viruses enter a host cell in a variety of ways, including fusion and endocytosis.

Many enveloped viruses get inside the cell by fusion : the virus envelope fuses with the cell membrane, and only the virus genome (nucleocapsid), which is enveloped by its protein coat, enters the cytoplasm.

Penetration via endocytosis can be observed for some enveloped and many non-enveloped viruses. The cell membrane pulls itself over the virus bound on the outside – a small membrane bubble (vesicle) is formed, which is pinched off into the cell interior. There the virus leaves the sac and begins to use the cell’s enzymes to multiply.

3. Release of the virus genome (uncoating)

The virus genome is now “stripped” inside the host cell. This means that the viral nucleic acid (RNA or DNA) is released from its protein coat (capsid) and any virus envelope that may be present. Depending on the type of virus, this process can be more or less complicated and is not yet precisely known for all types.

4. Multiplication (replication)

The virus genome is now duplicated and “read” – the cell’s own protein synthesis machinery produces the various virus components (virus proteins) based on the blueprints stored in the virus genome. In detail, these processes show some differences in different virus types:

>> Replication of DNA viruses :

The DNA viruses normally smuggle their DNA into the cell nucleus, where it is replicated. In addition, the same thing happens there with the viral DNA as with the cell’s own DNA: It is transcribed into mRNA (messenger or messenger RNA) – a kind of copy of the blueprints for the various virus proteins stored in the virus DNA. The viral mRNA molecules leave the cell nucleus and are “read” in the cytoplasm for the production of viral proteins.

Among the various DNA viruses, the poxviruses play a special role in terms of their replication: they are the only DNA viruses whose replication takes place entirely in the cytoplasm.

>> Replication of RNA viruses:

With RNA viruses, replication takes place in the cytoplasm (except for influenza viruses and Borna viruses – like most DNA viruses, they multiply in the cell nucleus). Replication of a virus with double-stranded RNA is relatively simple: the viral RNA can be used directly to produce the viral proteins.

In the case of viruses with single-stranded RNA , this is only possible if the RNA strand is so-called positively oriented. Such viruses are called (+)-strand RNA viruses (or (+)-sense RNA viruses). Their RNA genome is usually “read” directly as viral mRNA for the production of viral proteins.

This is different with the so-called (-)-strand RNA viruses (or (-)-sense RNA viruses), whose genome consists of a single negatively oriented RNA strand. This must first be converted into a positively oriented RNA strand using special enzymes (which the virus itself brings with it). Because only this can be “read” by the cell’s own protein synthesis machinery.

A special replication method can be found in the so-called retroviruses (such as the AIDS pathogen HIV). These also belong to the (+) sense RNA viruses, but replicate in a different way:

Retroviruses have special enzymes with which they can convert their single-stranded (+) sense RNA into double-stranded DNA and incorporate this into the nucleus of the human host cell (also double-stranded DNA). Of course, this is more complex than using the single-stranded (+) sense RNA directly for protein production (like other (+-) sense RNA viruses). However, this method had a lasting advantage for retroviruses: the daughter cells that are formed each time the host cell divides also carry the viral DNA in their genome and produce new viruses without ever having been infected by the retroviruses themselves.

The following applies to all viruses: the viral mRNA displaces the cell’s own mRNA from the protein synthesis machinery of the host cell to a certain extent. So the cell has no choice but to prefer to produce virus proteins instead of its own proteins.

5. Assembly

The assembly to “complete” viruses, consisting of the continuously produced copies of the virus genome and the new virus proteins, takes place either in the cell nucleus (in most DNA viruses) or in the cytoplasm (in most RNA viruses) or in the Cell nucleus and in the cytoplasm:

Each virus genome is wrapped in a protein coat (capsid). The “naked” (non-enveloped) viruses are done with it, while the enveloped viruses are still missing their envelope. How they get there varies.

For example, herpes viruses use the cell nucleus for this: the assembly of the new nucleocapsids takes place in the cell nucleus. When exiting the nucleus, the nucleocapsids then take part of the two-layer nuclear membrane (more precisely: the inner nuclear lamella) with them as an envelope. Many other types of enveloped virus, on the other hand, only acquire their envelope when they exit the host cell (see next point).

6. Release of the new viruses

In the case of many enveloped viruses, this happens through budding , whereby the newly formed nucleocapsids then also receive their envelope straight away: a newly formed virus genome with a protein coat (nucleocapsid) approaches the inside of the cell membrane, is enveloped by part of the membrane and released when it is discharged then off. So here a part of the host cell membrane – together with some virus proteins – becomes the virus envelope.

With some other enveloped viruses, which have already received their envelope inside the cell (eg herpes viruses), as well as with non-enveloped viruses, the release takes place through “bursting open” of the host cell ( cell lysis ).

Which host cells does a virus use?

The various viruses have each specialized on specific host organisms or host cells.

Human pathogenic viruses infect human cells , whereby the various virus types have more or less specifically adapted to certain cell types – more precisely to certain binding sites (receptors) on the outside of cells. For example, the AIDS pathogen (HIV) can only attack cells with so-called CD4 receptors – and these are exclusively certain white blood cells ( leukocytes ). All other body cells lack this surface protein so that HI viruses cannot penetrate them.

Other virus types, on the other hand, have specialized on receptors that are located in the same or very similar form on many different cell types – either from the same host species or from other species. Some viruses are able to use different types of living beings as host organisms.

The best-known example of this is the AIDS pathogen HIV. According to current knowledge, it originally only infected animal cells , namely those of primates (such as chimpanzees). At some point during the last century, the virus managed to spread to humans as well.

Some other viruses can also be transmitted from humans to (certain) animals and vice versa. Avian influenza , rabies and Ebola are examples of such viral zoonoses. Purely animal-pathogenic viruses, on the other hand, are (so far) limited to certain animal species as host organisms.

Plant cells can also be forced by some viruses to produce new pathogens. These plant-pathogenic viruses are the viroids described above – i.e. viruses that only consist of a ring-shaped RNA molecule (without a capsid or envelope).

There are also viruses that use bacterial cells as host cells. They are called bacteriophages .

How fast and how much do viruses multiply?

The period of time from the start of the uncoating phase to the appearance of the first infectious new viruses in the host cell is called the eclipse . It takes different lengths of time for different types of viruses – for example, about 30 hours for adenoviruses (important cold pathogens), eight to ten hours for retroviruses (like HIV), about five hours for herpesviruses and only about ten minutes for bacteriophages.

The number of viruses newly formed per host cell also varies considerably. For example, it is only 50 to 100 for the herpes simplex virus, but for the pathogen that causes polio (poliovirus) it is around 1,000. In the case of adenoviruses and retroviruses, around 1,000 new virus particles are also produced per host cell. In contrast, with picornaviruses, which include the most common cold viruses (rhinoviruses), around 100,000 new pathogens can develop in each host cell.

Viruses: treatment

The treatment of a viral infection depends on the type, severity and course of the infection.

antivirals

Virus-inhibiting drugs (virostatics) exist against a few types of pathogens . They can suppress the spread (multiplication) of viruses in the human body. However, the drugs cannot kill viruses (especially since viruses do not really “live”).

Antivirals are available, for example, against influenza – the so-called neuraminidase inhibitors. However, they usually only work if they are taken within the first two days after the onset of flu symptoms. They are also ineffective against some types of influenza viruses.

Scientists have also succeeded in developing effective antivirals against chronic hepatitis B , chronic hepatitis C and HIV . Ditto for herpes infections (like cold sores ).

However, such a causal treatment with antivirals is missing for the vast majority of types of viral infections. At least the symptoms can then be alleviated (symptomatic treatment), for example with antipyretic and anti-inflammatory drugs.

interferons

Interferons are endogenous messenger substances that are produced by many body cells in response to a viral infection (and other diseases). Among other things, they have an antiviral effect and thus play an important role in the immune response.

There are now also artificially produced interferon preparations that can be used as drugs against certain viruses. Such an interferon therapy is available, for example, against chronic courses of hepatitis B and C and against genital warts.

Do antibiotics help against viruses?

Doctors often prescribe antibiotics to treat bacterial infections. However, these drugs do not help with viruses. The reason for this lies in their mechanism of action: antibiotics can kill bacteria or prevent them from multiplying. To do this, they attack, among other things, the cell wall and the metabolism of the bacteria. Viruses have neither one nor the other. This is why antibiotics are ineffective against viruses.

If a doctor nevertheless prescribes antibiotics for a viral disease, this usually has another reason: the viral infection can weaken the body’s defenses to such an extent that a bacterial infection can also develop. Such bacterial superinfection can be treated or prevented with antibiotics.

viral diseases

Whether and what consequences a virus infection has varies greatly:

The immune system can very often completely eliminate invading viruses before they can multiply in the body and possibly cause symptoms. In some cases, the viruses multiply in the body, but those affected do not develop any symptoms ( asymptomatic infection ).

However, a viral infection often leads to an acute viral disease with more or less severe symptoms, which heal after some time (on their own or with treatment). However, there are also chronic viral infections (such as hepatitis B , hepatitis C or HIV) in which those affected shed viruses continuously and for a long time.

Latent viral infections are also possible , for example with HIV and herpes viruses: the genetic material of the viruses remains in the host cells after the acute infection without replicating – sometimes for many years. This latency phase can be accompanied by symptoms, but can also be symptom-free. Sometimes the “dormant” viruses are reactivated (eg by weakening the immune system). Then the virus replication starts and an acute illness breaks out – as in the case of AIDS. Another example is the rare subacute sclerosing panencephalitis. This is a chronic, progressive inflammation of the entire brain that can occur months to years after an acute measles infection.

Ways of transmission of a viral infection

Viral particles can enter the body in a number of ways. A distinction is made between the following transmission paths:

  • Droplet infection : Viruses that settle in the upper respiratory tract can be transmitted through tiny droplets when you sneeze or cough .
  • Aerosol infection: Similar to droplet infection, viruses can also be transmitted through aerosols. The ejected droplets have, however, a very small size. As a result, aerosols can float in the air for a long time and therefore spread over greater distances.
  • Smear infection : In the case of a smear infection, the viral pathogens are transmitted through direct contact. For example, when shaking hands or when reaching for contaminated objects (e.g. door handles), viruses can get on the hands and unconsciously be guided to the mucous membranes of the mouth , nose or eyes.

Important viral infections or viral diseases

Viruses can cause more or less serious diseases. Here is an overview of known virus infections:

  • Flu (Influenza) : Flu viruses are highly variable RNA viruses. Therefore, a new flu vaccine must be developed for each flu season – based on the virus types that will (probably) be responsible for most influenza cases in that year.
  • Common cold : The most common triggers are rhinoviruses, followed by adenoviruses and coronaviruses (see below).
  • other viral respiratory infections : For example, viruses are the main cause of acute pharyngitis and acute bronchitis. Inflammation of the larynx , tonsillitis and inflammation of the lungs (pneumonia) can also be viral.
  • Herpes infections : The most well-known include cold sores, genital herpes, chickenpox , shingles , cytomegalovirus , three-day fever and glandular fever .
  • Mumps : The virus infection, also known as goat peter, is often accompanied by painfully swollen parotid glands, but can also be symptom-free.
  • Measles : The highly contagious measles virus triggers a typical red skin rash .
  • Rubella : Rubella viruses can also cause a red rash. It is more small-spotted than that of measles.
  • Rubella : Despite the similar name, its cause is not the rubella virus, but the parvovirus 19. This is the smallest virus known to date that can cause diseases in humans.
  • Polio : This viral disease, also known as poliomyelitis (polio for short ) , no longer occurs or hardly ever occurs in many parts of the world thanks to widespread vaccination.
  • Smallpox : Thanks to a worldwide WHO vaccination campaign, the disease has been officially eradicated since 1980.
  • Rabies : Rabies viruses can be transmitted to humans through the bite of infected animals (such as dogs, foxes) and then infect the brain . Once the disease has broken out, it is always fatal.
  • Human papillomavirus (HPV) infection: Different types of these DNA viruses can cause different types of warts (e.g. common warts, genital warts ) and certain types of cancer ( e.g. cervical cancer , penile cancer, throat cancer ).
  • Molluscum contaginosum viruses are responsible for these “fake” warts. a representative of the smallpox virus.
  • Rotavirus and Norovirus Infections : Like norovirus , rotavirus is highly contagious and is responsible for many acute gastrointestinal illnesses associated with diarrhea and vomiting, particularly in children.
  • TBE : Tick-borne encephalitis is triggered by viruses that are transmitted by ticks.
  • Inflammation of the liver (hepatitis) : It is mostly caused by viruses, especially different types of hepatitis viruses. Other possible triggers of viral hepatitis are, for example, herpes viruses.
  • Hemorrhagic fever : The term includes various viral diseases that are associated with fever and increased bleeding tendency (and thus also internal bleeding). These include Ebola , Chikungunya , Yellow Fever , Dengue Fever and Lassa Fever .
  • HIV/AIDS : The acquired immunodeficiency disease AIDS is the final stage of an infection with HI viruses. It kills about 1.8 million people worldwide each year.

The corona viruses, which are mentioned as a common cause of colds, can trigger more or less severe respiratory diseases. However, the novel coronavirus Sars-CoV-2, which is currently rampant and which causes the disease Covid-19, is only distantly related to “regular corona viruses”. Rather, it is very similar to the SARS pathogen discovered in 2002.

Protection against viruses

Many viral infections can be prevented with general protective measures . Which these are depends on the way a virus is transmitted. Important measures include:

  • Hygiene: This includes regular thorough washing of hands, especially before eating and after using public transport and visiting public places (where you have touched doorknobs and railings, for example). Also important: Do not touch your face, mouth or nose with your (unwashed) hands.
  • Distance: Avoiding contact with infected or sick people: Many viruses are mainly transmitted via virus-containing droplets that infected people emit when they cough, sneeze and speak (e.g. flu viruses, measles viruses).
  • Food hygiene: For example, noroviruses can be transmitted via raw food and drinking water.
  • Safe sex : Some viruses are transmitted during sexual intercourse, such as HIV, genital herpes and HPV types, which promote the development of genital warts and cervical cancer, among other things.
  • Insect repellent: Mosquito repellents can protect against viral diseases that are transmitted by mosquitoes (e.g. yellow fever in tropical-subtropical regions). Tick ​​repellents prevent TBE. General measures to prevent stings or bites from blood-sucking insects (such as wearing long trousers and long sleeves) are also advisable.

How long do viruses survive outside the body (e.g. on doorknobs)? This depends heavily on the type of virus and the environmental conditions such as temperature, humidity and type of surface. For example, human corona viruses (such as the Covid-19 pathogen) are not very stable on dry surfaces. On drying, they are usually inactivated within hours to days.

Vaccinations also offer effective protection against various viruses . For example, a combined vaccine against measles, mumps and rubella (MMR vaccine) is available. This can also be expanded to include a vaccine against chickenpox (MMRV vaccination). There is also a new flu shot every year . TBE vaccination , shingles vaccination , HPV vaccination , hepatitis vaccination , rotavirus vaccination and rabies vaccination are also available.

Such vaccines are used for active immunization . So they stimulate the immune system to produce specific antibodies against the virus in question. Passive immunization is also possible against some viral infections . In this case, the body is already given “ready” antibodies against certain viruses, which offer temporary protection. This passive immunization can be carried out before a possible infection (e.g. hepatitis A ) or shortly after infection with certain viruses (e.g. rabies, hepatitis viruses) are given.

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