Viruses are complex macromolecules (giant molecules) that can only replicate in living cells (host cells). They can be spread outside cells by transmission. Viruses consist of a protein envelope that contains genetic information. Viruses have been around for several million years – the oldest evidence of their existence was discovered in dinosaur bones.

Particle sizes of viruses

Viruses are organic structures that are not visible to the naked eye. With diameters of approximately 0.015 µm to 0.4 µm (µm = micrometer), virions (=individual virus particles outside a cell) are assigned to the PM1 category. The PM10, PM2,5 and PM1 categories classify particles according to their size. This now globally established classification was originally defined by the U.S. Environmental Protection Agency (EPA). This classification is particularly important for effective air filtration in order to select the appropriate filter according to the size of the particles that need to be filtered.

Virus size comparison

Viruses as pathogens

Pathogens are microorganisms or subcellular agents that can affect the health of other organisms. Medicine also refers to them as germs. Examples of pathogens include viruses, bacteria, parasites, fungi and algae. Viruses contain programs that enable them to multiply and spread. However, they are unable to reproduce independently and they do not have their own metabolism. For this reason, they cannot obtain energy either by ingesting food or via photosynthesis. Strictly speaking, therefore, viruses are not living organisms.

However, due to their ability to reproduce and evolve, they are considered “close to life”. They require a host cell to reproduce, such as from plants, animals or humans. In the host cell, they introduce their own genetic information. In this way, the host cell’s genetic material is programmed to produce even more viruses. Although viruses behave in a similar way to parasites, they are not classified as such because the latter are classified as living organisms. Viruses are instead classified as obligate intracellular parasites, since they behave parasitically inside the cell but are lifeless outside it.

Virus transmission – droplet infection

How can viral infections be treated?

Although viruses have a very simple structure, they are extremely effective because they undergo change (viral mutations) and constantly develop new defense mechanisms. Many viral infections heal spontaneously and do not require therapy. Some viral infections, on the other hand, run chronically over an extended period of time. These include hepatitis C or HIV infections, for example. Antibiotics are only effective for bacterial infections, but not for viruses. They can attack the cell wall of bacteria and thus destroy them. In addition, bacteria have a metabolism that antibiotics can interfere with. Viruses, however, have neither a cell wall nor a metabolism. 

Medications: Antivirals are drugs that are used against viruses. These active antiviral agents prevent the viruses from docking with the body’s own cells or prevent them from replicating. In this way, antiviral drugs are able to shorten the duration of the disease. However, antivirals can only combat individual types of virus. HIV medications, for example, work in this way, allowing those affected to achieve an almost normal life expectancy. Much like bacteria develop resistance to antibiotics, viruses can change through mutation, potentially eliminating the effect of antiviral drugs. In the case of viral infections for which no drugs exist, the focus is on alleviating symptoms, for example with pain-relieving or fever-reducing agents.

Vaccinations: Vaccinations are a measure to prevent viral infections. Various infectious diseases have been virtually eradicated thanks to a vaccination campaign, for example smallpox, tuberculosis or polio. The basis for the effect of vaccinations is the body’s own defense mechanism, known as the antigen-antibody reaction. The administration of the vaccine causes our immune system to form antibodies against the pathogen. In the event of infection with the antigen, the organism uses these antibodies to prevent an outbreak of the disease. High vaccination coverage rates also provide immunity to unvaccinated individuals, since the pathogen can only spread with much greater difficulty (= herd immunity). Since viruses continue to evolve, regular new vaccines are needed against influenza, for example, which have been adapted to the mutated viruses.

How can viruses spread?

Viruses can only reproduce in living cells. Outside of these, they are passed on via transmission. There are several ways in which people can become infected with viruses: 

Contact infection: In the case of a (direct) contact infection, the pathogens are transmitted through direct physical contact, for example when shaking hands.

Smear infection: Pathogens can become attached to objects. Contact with surfaces therefore makes it possible for viruses to be transmitted. If you touch an infected object, such as a door handle, and then touch your face, virus particles can enter your body through your mouth, nose or eyes. Smear infection is also referred to as indirect contact infection. 

Droplet infection: In this case, transmission takes place via airborne aerosols. Small droplets carrying virus particles enter the air when sneezing, coughing, talking and even breathing. The smallest droplets (=aerosols) can remain suspended in the air for a long time and be absorbed by other people as they breathe.

The risk of infection with viruses is particularly high where there are many people in a confined space. This is where the viruses are transmitted, for example, via door handles (contact infection). But viruses can also be airborne. In heavily frequented buildings, they are sometimes distributed by the ventilation system and can thus endanger a large number of people. Likewise, large numbers of people are in close proximity to each other in vehicles or on public transportation, which increases the risk of infection.

Evaporation of a liquid expelled droplet to a droplet-nucleus

Examples of known viruses

Rhinoviruses are germs that in most cases are the cause of colds and runny nose. It is not the viruses themselves that trigger symptoms such as coughing, sneezing or headaches, but rather the body’s own immune defense system that is largely responsible.

More than 100 different types of viruses are counted among the herpes viruses. The best-known herpes disease is lip herpes (herpes labialis) with painful blisters. However, viruses from the herpes family are also responsible for chickenpox or shingles. In the case of a latent infection, the patient shows no signs of illness, but can transmit viruses.

Influenza viruses damage the mucous membrane of the respiratory tract, weaken the immune defense and lead to “real flu”. This is one of the most common viral infections. A special feature of influenza viruses is their variability, i.e. their individual components change regularly. For this reason, an infection that has been overcome does not provide immunity if the influenza virus has already changed. 

Measles viruses are transmitted via droplet infection. Measles is one of the typical childhood diseases and one of the most contagious infectious diseases. A measles infection can result in severe complications. However, thanks to measles vaccination, cases have become rare.

TBE (early summer meningoencephalitis) is also triggered by a virus that is usually transmitted by ticks. The viruses can enter the human bloodstream through a tick bite. However, not every bite of an infected tick leads to infection. TBE risk areas in Germany have the ideal humidity for ticks. 
The HI virus (HIV) attacks crucial cells within the immune system – known as T-helper cells – and thus weakens the body’s defenses. The onset of AIDS means that the body is no longer able to fight invading pathogens such as bacteria, fungi or viruses. Although drugs cannot cure AIDS, they can suppress the spread of the HI virus. 

Coronaviruses were discovered as early as the 1960s and can infect both humans and animals. SARS-CoV-2 is a new coronavirus that was identified in early 2020 as the trigger of the respiratory disease COVID-19. The smallest virus particles remain in the air for a long time, where they can be easily inhaled.

How do you protect yourself and others from viruses?

Viruses are ubiquitous and often harmless. Nevertheless, some of them pose a considerable health risk. There are various measures you can take to protect yourself and others from infection. These are particularly important in a pandemic.

Hygiene: Hygiene rules are an important pillar for preventing infections. Regular hand washing is essential to prevent contact infections. Hands should be soaped thoroughly for at least 20 seconds and then dried carefully. If you are out and about and do not have the opportunity to wash your hands, you should avoid touching your face with your hands. When coughing or sneezing, the crook of the arm or a handkerchief should be used.

Mouth-nose protection: Wherever several people are gathered in a confined space, the wearing of a mouth-nose protection is recommended. This applies, for example, to public transportation or buildings. The various types of masks differ not only visually, but also, and above all, in their protective effect. Everyday masks made of fabric can only reduce droplet ejection. Medical face masks (surgical masks) protect other people and offer the wearer a low level of protection against aerosols themselves. FFP masks (particle filtering half masks) provide the best protection – they retain a large proportion of the smallest aerosol particles.


Personal distance: The closer you get to other people, the higher the risk of coming into contact with aerosols containing the virus. Therefore, it is best to refrain from hugging or shaking hands with others. Avoid larger crowds if possible. In addition, maintain a minimum distance of 1.5 meters from other people. 

Fresh indoor air: The risk of infection is particularly high in poorly ventilated indoor areas. A regular supply of fresh air can reduce the spread of viruses via aerosols. If the indoor air is exchanged with fresh outdoor air during ventilation, the concentration of aerosols in the room decreases. However, leaving the windows tilted is not sufficient for air exchange. If you want to prevent the spread of viruses in the room, you should ventilate intermittently with the windows wide open. But ventilation alone is often not enough. Effectively lowering the aerosol concentration can be achieved with the help of air-conditioning and ventilation systems that use a high proportion of outdoor air.

Effective virus protection thanks to air filters

Viruses are not able to distribute themselves independently in the room. That is why they adhere to droplets or particles (= aerosols). Suitable air filters have the function of separating a high percentage of aerosols with adhering viruses and germs. Air filters are therefore an important measure for reducing the risk of infection indoors. For optimum indoor air quality, the combination of air filters of different filter classes and designs to form multi-stage systems has proven effective. This allows the most efficient filter solution to be matched to the specific requirements. EPA, HEPA and ULPA filters are rated according to DIN EN 1822-1:2019 or ISO 29463-1:2017.

HEPA filters
In sensitive areas, additional integrated HEPA filters in the final filter stage are indispensable. These are standard in cleanrooms in the pharmaceutical and food industries or in hospitals. HEPA filters reduce the risk of infection by retaining at least 99.995 percent of particles and viruses. Separated viruses remain in the filter, where they decompose over time. Intelligent filter design prevents the filters themselves from becoming a source of contamination. Based on the European filter standard EN 1822, the new ISO 29463 plays an important role in the testing of HEPA filters.

Cabin filters for vehicles and buses
The risk of becoming infected is not only higher in buildings, but also in vehicles or buses. An increased supply of fresh air minimizes the risk of virus transmission in these places as well. Suitable automotive cabin air filters are able to greatly reduce the concentration of virus aerosols. At the same time, they prevent the release of active viruses back into the cabin air.
The cabin air filters in the micronAir proTect line provide active protection by significantly reducing the concentration of virus aerosols. This effect is achieved by a unique structure with multiple filter layers. They are capable of effectively filtering, trapping and preventing harmful environmental gases as well as inorganic and biological particles and aerosols. Thanks to their high filtration performance level for particles and viral aerosols, the concentration of viral aerosols in the air can be significantly reduced. Automotive cabin air filters from the micronAir proTect line thus make a significant contribution to minimizing the risk of virus transmission and protecting occupants inside the vehicle.

Clean air in living areas for a healthier life
Poor indoor air quality with a high concentration of pollutants can affect our health. Complete filter solutions ensure safer and healthier living spaces. Our high-performance air cleaners remove airborne mold, pet hair and dust mites from living spaces. Multi-stage filtration systems provide effective filtration for healthy indoor air to protect people and save energy.

Your contact for virus-free indoor air

Freudenberg Filtration Technologies is your point of contact for reducing infection risks and achieving healthy indoor air in interiors and vehicles. We not only offer actual filtration products, but also provide you with tailor-made service solutions. Ventilation systems that are ideally maintained, hygienically sound and equipped with highly effective air filters make an important contribution to infection control. We also ensure healthy indoor air in vehicles and buses with highly effective automotive air filters.

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Technical filter information for protection against viruses and germs