CORONA PANDEMIC: HOW VIRUSES REALLY WORK

As the topic of viruses is very popular due to the current corona pandemic, some general factual background information about viruses is covered here. From a biological perspective, it is exciting to understand more precisely what processes the small particles in our body bring about. Without our cellular mechanisms, they could not spread at all, which is to their advantage, as they do not damage our organism too much. They could also bring us medical benefits, for example in cancer research.

What is a virus? Why are viruses not living beings?      

By official definition, living beings consist of cells. They carry out an active metabolism, grow, reproduce, and react to stimuli¹. Viruses are not cells and therefore have no metabolism of their own, because their existence consists merely of a protein capsule containing genetic information. Although some viruses have an envelope around their capsule that resembles a cell membrane, this still does not allow them to carry out an independent metabolism². A virus therefore cannot eat and excrete waste products, like animals and humans or even bacteria in a primitive form. By the way, viruses are not therefore referred to as parasites, as the term is limited to living beings, so it is more commonly used for bacteria, single-celled organisms, and worms, which are harmful to the affected organism.

How do viruses reproduce?    

The goal of viruses is to maintain their own existence. To this end, it is of great advantage to reproduce extensively as quickly as possible. Since this, as explained above, cannot happen independently, a foreign cell is needed as a so-called host . For viruses, these are primarily humans and animals, or rather their cells. A virus therefore absolutely needs cells from a living being to settle there and use the host cell for its own reproduction².       
There are therefore mainly two mechanisms that the pathogen uses: on the one hand, the transmission from one host to another, on the other hand, the passive reproduction within the respective host.

Step 1: Transmission

There are various ways through which a virus can get from one host to another:

1. Aerosol droplet infection:            
   The viruses are transported out into the air via tiny respiratory droplets (lungs, nose, mouth). There they usually fly invisibly in the immediate vicinity of the carrier (for example, a person). Especially sneezing and coughing release many of these respiratory droplets. Because they are so light, they can linger there for several minutes, and their range is 1-3 meters depending on the type³. Examples of this mode of transmission are classic influenza (flu) viruses and the currently widespread SARS-CoV-2, which can cause corona disease.
2. Direct transmission:  
   In this case, a virus is passed on through direct saliva flow or body contact. For example, the herpes virus, which can cause fever blisters on the lip, is transmitted by kissing. For this virus, this only happens at the time of the outbreak, i.e., when the symptoms are currently occurring².
3. Sting or bite of an animal:
   Viruses can also spread with the help of a so-called vector, a carrier. The animal itself or its cells only serve to bring the viruses to the actual host, without the vector itself getting sick. A popular example of this is the rabies virus, which causes the disease rabies. Between 1989 and 2011, many people were infected through animal bites².
4. Body fluid or blood during sexual intercourse:        
   Viruses that are transmitted in this way have optimal growth conditions when they are exposed to no light but plenty of moisture. During sexual intercourse, they are transmitted through the body fluids exchanged there. They are also found in the blood, and through a minor cut or injury, there is another route. HIV, the virus that causes AIDS, is the prime example of this².  
5. Fecal-oral:      
   The viruses are transported out through feces and taken in orally by the next host. In poor hygienic conditions, such as those known from earlier times or from third-world countries, this route of transmission is conceivable. With clean toilets and regular hand washing after using the toilet, this should not happen. Hepatitis A viruses use the fecal-oral route of transmission².
6. Indirect contact infection:    
   The surfaces of objects serve as a platform for the viruses, on which they linger until another host comes along and touches the object. One then only has to unconsciously touch their face, and they have already taken in the virus.

For many viruses, transmission can occur through several of the mentioned routes. The current virus SARS-CoV-2, for example, spreads through aerosol droplet infection, direct transmission even during hugs, and indirect contact infections on surfaces.

Step 2: Reproduction        

After a virus has been transported from one host to another through one of the mentioned routes, it uses the mechanisms of the host cells for its own reproduction. This process is described using an infected human cell:    
Our cells have a cell nucleus containing the so-called DNA, on whose strands our genetic information is located. This enormous information store is constantly used for metabolic processes and everything that happens in our body. Most cells divide in the normal cell cycle to destroy old ones, so that we can grow and stay healthy. In addition, small sections of the DNA are copied to the so-called RNA, which transports the transcribed information out of the cell nucleus. There it is used to give instructions to the cell organelles for specific metabolic processes. All of this is relatively complex and abstract; for the reproduction process of viruses, it is enough to know that we have important information on the DNA and the RNA.

Viruses also contain such information in their capsule. This is present either as DNA or as RNA.  The virus uses proteins located on its surface, which can dock to receptors on the respective human cells. Our cells usually cannot recognize that it is a foreign body and open the gate. Thus, the viruses give their genetic information to our cells, and these then start to copy this actually foreign DNA or RNA and follow the "instructions". This causes our cell to assemble several copies of the respective virus, which thus reproduces as quickly as possible. One can imagine this intrusion like a Trojan horse that sneaks in the villains unnoticed².

To what extent are viruses important for the human organism?   

Many viruses known to us can cause symptoms and diseases in us. For example, the Human Immunodeficiency Virus (HIV) infects important cells of our immune system, whose function is then disabled. In healthy people, the infected T-helper cells protect the organism from pathogens such as bacteria, other viruses, fungi, etc. As soon as they can no longer perform this job, we are extremely susceptible to disease. An acquired immunodeficiency syndrome is created, which is also known as AIDS. This leads to the fact that those affected get follow-up diseases, such as tuberculosis or pneumonia, which can no longer be fought by the immune system. This can ultimately end fatally without the right medication⁴. 
But not all viruses are a danger to us, and they may even help us cure diseases in the future:

Viruses, evolution, and their benefit for humans   

Scientific studies have shown that a considerable part of our human genetic material comes from viruses. They have obviously already built themselves into our family tree in the apes and later also in the modern human species in the DNA. Researchers assume a viral share of 8–43%⁵. Even if only the smaller number were true, it is obvious that viruses have played a central role in which genome sequences are present in our genetic material. Since evolution is based on the "survival of the fittest" principle, these originally foreign sections probably did not harm us. They may have even made a positive contribution to our development.        
Certain viruses could have another benefit for our healthcare system: Viruses are currently being programmed to attack cancer cells. Cancer researchers have made use of the fact that the genetic material of a virus can almost unnoticed enter our cells and change their gene activity. Certain viruses can be weakened and modified in the laboratory so that they can very specifically kill tumor cells or inhibit their activity. This is associated with fewer complications compared to chemotherapy, because chemotherapy also damages healthy cells⁶. Research into these modifications of viruses in cancer treatment is still ongoing, but it gives us hope for better support in healing.

Conclusion

Viruses are tiny particles that do not belong to any living being. They need a host to reproduce and further hosts to spread. There are many viruses that can cause diseases, many that do nothing to us, and possibly also some that are useful for humans.          

How we can protect our body and our immune system from pathogens such as viruses, and what our own defense cells do daily, will be explained in the next article.

¹ Doccheck Flexikon: Living beings. [https://flexikon.doccheck.com/de/Lebewesen; 25.03.2020].

² Bütikofer, Markus; Hopf, Zensi; Rutz, Guido; Stach, Silke and Grigoleit, Andrea (2015): Human Biology 1: Fundamentals, Metabolism, and Defense Systems. Zurich: Compendio Bildungsmedien. pp. 139-141, 242.

³Dochcheck Flexikon: Droplet infection. [https://flexikon.doccheck.com/de/Tr%C3%B6pfcheninfektion; 28.03.20].

⁴Zschokke, Samuel (2018): Human Biology I: Anatomy and Physiology – Infectious Diseases. Basel: University of Basel.

⁵Heinemann, Pia (2016): How Viruses Sneaked into the Human Genetic Material. In: Die Welt. Berlin: Axel Springer SE.

⁶Till, Ulrike (2018): Fighting Cancer with Viruses. SWR.