Viruses

Martina Feichter studied biology with an elective subject pharmacy in Innsbruck and also immersed herself in the world of medicinal plants. From there it was not far to other medical topics that still captivate her to this day. She trained as a journalist at the Axel Springer Academy in Hamburg and has been working for since 2007 - first as an editor and since 2012 as a freelance writer.

More about the experts All content is checked by medical journalists.

Viruses are important infectious agents that can cause more or less serious illness. Infection occurs depending on the type of pathogen, for example through droplet, aerosol or smear infection, through contaminated food and during sex. Read everything you need to know about infectious agents here: What is a virus? What viral diseases are there? How do new viruses develop in the body? How can viruses be fought?

ICD codes for this disease: ICD codes are internationally recognized codes for medical diagnoses. They can be found, for example, in doctor's letters or on certificates of incapacity for work. B26B05J00B02J09A82B17B06B24B00

Brief overview

What are viruses Very small infectious particles that are among the most important pathogens causing 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 on their own - but bacteria can.
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. cold, sore throat, tonsillitis, bronchitis, pneumonia, flu, mumps, measles, rubella, rubella, smallpox, polio, herpes, virus-related liver inflammation (viral hepatitis), TBE, rabies, HIV, SARS, Covid -19
Virus infection - duration: Acute infections can subside after days to weeks. Chronic infections can last months to years (even lifelong).
What helps against viruses? Virus-inhibiting agents (antivirals) are available against some viral infections. Otherwise, only the symptoms of a viral disease can be treated (e.g. with antipyretic, pain reliever and anti-inflammatory drugs).

Viruses: definition & structure

Viruses are relatively simple biological structures and - besides bacteria - the most important causative agents of infectious diseases in humans. Not all viruses can attack human cells, however, and those that do don't always make us sick - often the immune system can get the invaders under control before the viral infection turns into disease.

Viruses are quite variable in shape (e.g. 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) under a light microscope. They are only visible in the electron microscope.

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

Viruses: build-up

The structure of viruses is quite simple. As a rule, a virus consists of its genetic material with a protein coat (capsid) and sometimes also a shell:

Genome: The virus genome is made up of double or single-stranded nucleic acid in connection with certain proteins. A nucleic acid is a chain-like molecule that, depending on its sugar component, is either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA). Accordingly, a distinction is made between DNA viruses (such as herpes, smallpox or adenoviruses) and RNA viruses (such as flu, hepatitis, measles, corona and HI viruses).
Capsid: This is the protein coat around the virus genome. Capsid and genome together are called the nucleocapsid.
Shell: Some viruses still have an outer shell made of a lipid bilayer.It comes in part from the cell membrane of the host cell (also a lipid bilayer) in which the virus in question originated. Special receptor proteins ("spikes") can be part of the virus envelope. They are used to attach the virus to a host cell.

Examples of enveloped viruses are influenza viruses and HI viruses. The "naked" viruses - viruses without a shell - include adenoviruses (common cold pathogens) and human papilloma viruses (HPV).

The simplest of all viruses are viroids. They consist only of their genetic make-up (a ring-shaped RNA molecule) - without a capsid or shell. Viroids attack plant cells.

Differences Between Viruses and Bacteria

Bacteria and viruses are both infectious pathogens. However, they differ from one another in several ways:

Nucleic acid: The genome of bacteria always consists of DNA, that of viruses, on the other hand, usually consists of RNA, less often of DNA.
Cell plasma (cytoplasm): Bacteria are made up of a cell with cytoplasm inside (like human and animal cells) - viruses are not.
Multiplication: Almost all bacteria have their own metabolism and can multiply on their own. This does not apply to viruses - they depend on host cells to reproduce (see below).

A metabolism of its own is one of the criteria that, by definition, make up living beings. Because viruses have no metabolism, they are not considered living beings.

Viruses: reproduction

Viruses are characterized by a parasitic behavior - that is, they need host cells in order to be able to multiply. 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 produce new viruses. After their release from the cell, these newly generated virus particles can in turn attack other cells - the virus replication cycle then begins anew.

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

Attachment (adsorption) to a host cell
Penetration into the host cell
Release of the virus genome (uncoating)
Multiplication (replication) of viruses
Assembly of the new viruses
Release of the new viruses

1. Attachment (adsorption)

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

2. Penetration

Viruses enter a host cell in various ways, such as fusion or endocytosis.

Many enveloped viruses get into the cell interior by means of fusion: the virus envelope fuses with the cell membrane, and only the virus genome (nucleocapsid) enveloped by its protein coat passes into the cytoplasm.

Penetration via endocytosis can be observed in some enveloped and many non-enveloped viruses. Here, the cell membrane turns over the virus bound on the outside - a small membrane vesicle is created that is pinched off inside the cell. There the virus leaves the vesicle 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 coat that may be present. This process can be more or less complicated depending on the type of virus and is not yet exactly known for all species.

4. Propagation (replication)

The virus genome is now reproduced and "read" - the cell's own protein synthesis machinery uses the blueprints stored in the virus genome to produce the various virus components (virus proteins). In detail, these processes show some differences in different types of viruses:

>> Replication of the DNA viruses:

The DNA viruses normally smuggle their DNA genome into the cell nucleus, where it is replicated. In addition, the same thing happens with the viral DNA as with the cell's own DNA: It is transcribed into mRNA (messenger or messenger RNA) - in a sense, a copy of the blueprints stored in the virus DNA for the various virus proteins. The viral mRNA molecules leave the cell nucleus and are "read" in the cytoplasm to produce the virus proteins.

Among the various DNA viruses, the smallpox viruses play a special role in terms of their replication: They are the only DNA viruses whose reproduction takes place completely in the cytoplasm.

>> Replication of the RNA viruses:

In the case of the RNA viruses, replication takes place in the cytoplasm (except for the flu viruses and Borna viruses - like most DNA viruses, they multiply in the cell nucleus). In the case of a virus with double-stranded RNA, it is relatively easy to replicate: 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 it is a so-called positively-oriented RNA strand. Such viruses are called (+) - strand RNA viruses (or (+) - sense RNA viruses). Your RNA genome is usually "read" directly as viral mRNA for the production of the viral proteins.

This is not the case 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 the cell's own protein synthesis machinery "read".

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

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

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

5. Assembly

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

Each virus genome is packed in a protein coat (capsid). The "naked" (non-enveloped) viruses are done with it, while the enveloped viruses still lack their envelope. How they come to this is different.

For example, herpes viruses use the cell nucleus for this purpose: 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 a covering. Many other types of enveloped viruses, on the other hand, only receive their envelopes when they exit the host cell (see next point).

6. Release of the new viruses

In many enveloped viruses, this happens through budding, whereby the newly formed nucleocapsids then also receive their shell: 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 loosens when it is released then off. So here part of the host cell membrane - together with some virus proteins - becomes the virus envelope.

In the case of some other enveloped viruses that have already got their envelope inside the cell (e.g. herpes viruses), as well as with non-enveloped viruses, the release occurs by "bursting" the host cell (cell lysis).

Which host cells does a virus use?

The various viruses have each specialized in certain host organisms or host cells.

Viruses pathogenic to humans attack human cells, whereby the different virus types have adapted more or less specifically 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 HIV viruses cannot penetrate them.

Other types of viruses, on the other hand, have specialized in 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. For example, 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 the current state of 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 flu, rabies and Ebola are examples of such viral zoonoses. Purely animal pathogenic viruses, on the other hand, have (so far) been limited to certain animal species as host organisms.

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

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 from the beginning of the uncoating phase to the appearance of the first new, infectious viruses in the host cell is called an eclipse. It takes different lengths of time for different types of virus - for example, around 30 hours for adenoviruses (important cold pathogens), eight to ten hours for retroviruses (such as HIV), around five hours for herpes viruses and only around ten minutes for bacteriophages.

The number of viruses newly formed per host cell also varies considerably. In the case of herpes simplex viruses, for example, it is only 50 to 100, in contrast to the causative agent of polio (poliovirus), around 1,000. Adenoviruses and retroviruses also produce around 1,000 new virus particles per host cell. In contrast, picornaviruses, which include the most common cold viruses (rhinoviruses), can create around 100,000 new pathogens in each host cell.

Viruses: treatment

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

Antivirals

Antiviral drugs (antivirals) exist against a few types of pathogen. They can suppress the spread (reproduction) of viruses in the human body. However, the drugs cannot kill viruses (especially since viruses do not really "live").

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

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

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

Interferons

Interferons are the body's own messenger substances that are produced by many cells in the body 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 manufactured interferon preparations that can be used as drugs against certain viruses. Such 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 for 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 the cell wall and the metabolism of the bacteria, among other things. Viruses have neither one nor the other. That is why antibiotics are ineffective against viruses.

If a doctor still prescribes antibiotics for a viral disease, there is usually another reason: the viral infection can weaken the body's defenses so 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:

Very often the immune system can completely eliminate viruses that have entered the body before they can multiply in the body and possibly cause symptoms. In some cases, the viruses multiply in the body, but people do not develop any symptoms (asymptomatic infection).

However, a viral infection often leads to an acute viral disease with more or less severe symptoms that heal after a while (alone or under 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 virus infections are also possible, for example in the case of HIV and herpes viruses: After the acute infection, the genetic material of the virus remains in the host cells without replicating - sometimes even for many years. This latency phase can be accompanied by symptoms, but it can also be symptom-free. Sometimes the "dormant" viruses are reactivated (e.g. by weakening the immune system). Then the virus replication gets going and an acute illness breaks out - as with AIDS, for example. 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.

Routes of transmission of a virus infection

Virus particles can enter the body in various ways. A distinction is made between the following transmission routes:

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

Important viral infections or viral diseases

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

Influenza: Influenza viruses are very 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 the most influenza cases 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 leading cause of acute sore throat and acute bronchitis. Laryngitis, tonsillitis and pneumonia can also be viral.
Herpes infections: The best known include cold sores, genital herpes, chickenpox, shingles, cytomegaly, three-day fever and Pfeiffer's 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 causes a typical red rash.
Rubella: Rubella virus can also cause a red rash. It has smaller spots than that of measles.
Rubella: Its cause is - despite the similar name - not the rubella virus, but the parvovirus 19. This is the smallest virus known to date that can cause illness in humans.
Polio: This viral disease, also known as poliomyelitis (polio for short), no longer or hardly occurs in many parts of the world thanks to extensive vaccination.
Smallpox: Thanks to a global WHO vaccination campaign, the disease has been officially eradicated since 1980.
Rabies: The rabies virus can be transmitted to humans through the bite of infected animals (such as dogs, foxes) and then affect the brain. Once the disease breaks out, it's always fatal.
Human papillomavirus (HPV) infection: Different types of these DNA viruses can cause different types of warts (e.g. common warts, genital warts) as well as certain types of cancer (such as cervical cancer, penile cancer, larynx cancer).
Dellular warts: Molluscum contaginosum viruses are responsible for these "fake" warts. a representative of the smallpox virus.
Rotavirus and norovirus infections: Like the norovirus, the rotavirus is highly contagious and is responsible for many acute gastrointestinal diseases with diarrhea and vomiting, especially in children.
TBE: Early summer meningoencephalitis is caused 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 encompasses various viral diseases that are associated with fever and an increased tendency to bleeding (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 end stage of infection with HI viruses. It kills around 1.8 million people worldwide each year.

The coronaviruses mentioned as a common cause of colds can trigger more or less severe respiratory diseases. The currently rampant new coronavirus Sars-CoV-2, which causes the disease Covid-19, is only distantly related to "regular coronaviruses". Rather, it is very similar to the SARS pathogen that was discovered in 2002.

Protection against viruses

Many viral infections can be prevented with general protective measures. Which these are depends on how a virus is transmitted. Important measures are for example:

Hygiene: This includes regular thorough hand washing, 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 put your (unwashed) hands on your face, mouth or nose.
Distance: Avoid contact with infected or sick people: Many viruses are mainly transmitted via virus-containing droplets that infected people emit when coughing, sneezing and talking (e.g. flu viruses, measles viruses).
Food hygiene: For example, noroviruses can be transmitted through raw foods and drinking water.
Safer sex: Some viruses are transmitted during sexual intercourse, such as HIV, genital herpes and HPV types, which among other things promote the development of genital warts and cervical cancer.
Insect repellent: Mosquito repellants can protect against viral diseases that are transmitted by mosquitoes, for example (e.g. yellow fever in tropical-subtropical regions). Tick repellants prevent TBE. General measures to prevent stings or bites from blood-sucking insects (such as wearing long pants 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 the type of surface. For example, human coronaviruses (such as the Covid-19 pathogen) are not very stable on dry surfaces. When 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 vaccination) is available. This can also be expanded to include a vaccine against chickenpox (MMRV vaccination). There is also a new flu shot every year. Also available are TBE vaccination, shingles vaccination, HPV vaccination, hepatitis vaccination, rotavirus vaccination and rabies vaccination.

Such vaccines are used for active immunization. So they stimulate the immune system to produce specific antibodies against the viruses in question. Passive immunization is also possible against some viral infections. The body is given "ready-made" antibodies against certain viruses, which offer temporary protection. This passive immunization can take place before a possible infection (e.g. hepatitis A) or shortly after infection with certain viruses (e.g. rabies, hepatitis) Viruses).

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