The Essentials
Viruses, bacteria, parasites, and their counterparts
Pathogens and Germs
The coronavirus (COVID-19) has been keeping the entire world, and us here in Switzerland, busy for well over a year now. As the name suggests, SARS-CoV-2 belongs to the category of viral infections. But what exactly are viruses? And how do they differ from bacterial infections and parasites? How are they treated? What actually works?
Transmission and Contagion
An infection can occur in several ways. The most famous and common is droplet transmission: when breathing, speaking, and especially when coughing or sneezing, viruses and bacteria attach to tiny droplets, are released into the air (aerosols), and are inhaled by others. This is how these pathogens are transmitted from person to person.
However, smear and contact transmission also pose a major risk. Here, pathogens are left behind on surfaces (door handles, handrails in public transport, etc.) and countless other people can become infected by touching these objects [1].
Pathogens can also be passed on through body fluids (blood, saliva, sexual fluids, etc.). Sexual intercourse can transmit various diseases, such as the bacterial pathogens of syphilis and gonorrhoea or the viral pathogens of herpes, hepatitis B, and HIV [1].
Furthermore, food and drinking water still rank among the sources of infection, particularly in poorer countries. In food, this includes salmonella (bacteria) or hepatitis A pathogens (virus), which cause diarrhoea. The most well-known example of drinking water as a source of infection is cholera (a bacterium).
While some pathogens can only be transmitted in one specific way, others can be transmitted in multiple ways. The best example of this is E. coli bacteria. They can be transmitted via contaminated food (especially meat), contaminated water, and even from person to person [1].
Some infectious diseases are passed on exclusively from person to person (e.g. measles). Diseases that are transmitted from animals to humans are called zoonoses [1]. The most common animal vectors are insects. The Anopheles mosquito, as the actual host of malaria (a parasite), as well as the yellow fever mosquito and Asian tiger mosquito (which transmits dengue fever and other life-threatening tropical fevers), are among the best-known sources of zoonoses. But insects are also potential pathogens in our latitudes: ticks can transmit early summer meningoencephalitis (TBE, a viral infection), a sometimes life-threatening meningitis, or borreliosis (synonym: Lyme disease, a bacterial infection) [2].
Next to insects, birds are the second most important transmitters of zoonoses (avian flu).
Infected Does Not Mean Sick
Whether a virus or bacterium attacks the body, the immune system responds with a complex and sophisticated defense reaction: the foreign cells are attacked.
If the body wins relatively easily and quickly, resolving the situation without much fuss, you won't get sick and won't feel much of anything.
Important: you may still be contagious under certain circumstances, because you are carrying the disease even though you haven't fallen ill from it!
Under certain circumstances, if the pathogen load is too large or too complex for the body's defense mechanism (it doesn't happen fast enough) or if the body detects them too late, the pathogens can spread and infect the body: you get sick and experience symptoms. A fever boosts the defense reaction, and as soon as the pathogens are back under control and decimated, you feel better and recover.
It's impossible to say exactly who gets sick, when, and why—because not everyone falls ill! This depends heavily on your specific, individual immune system, as well as the number of pathogens (germ load) and the danger level of the pathogen.
Viral Infections
US government, Infectious bronchitis virus, marked as public domain, details on Wikimedia Commons
Viruses are by far the most significant pathogens causing upper respiratory tract infections and diarrheal illnesses.
Viruses are very simply structured particles consisting only of a strand of genetic information (DNA/RNA) and a few protein cells. Because of this, viruses do not have their own cells or their own metabolism—meaning they are not living organisms in their own right.
Schematic Structure of a Virus:
Y_tambe, Virion, CC BY-SA 3.0
Viruses invade animal, plant, and human organisms, searching for a host cell to replicate. However, certain viruses can also survive on surfaces for some time. If they do not find a host cell to replicate in, they can eventually die off. It's important to note that not all viruses that infect humans actually make us sick. Our immune system often reacts very quickly and fights off the pathogens successfully [3].
As soon as a pathogen enters our body, we are infected and can transmit the virus to others—EVEN WITHOUT SHOWING SYMPTOMS OR FEELING SICK! In the case of viral infections, this often happens through close direct contact, especially in enclosed spaces, via coughing, sneezing, or, for example, shaking hands. But it also happens through indirect contact, such as door handles, handrails on buses/trams/trains, etc., because, as mentioned, viruses can survive on surfaces for some time.
The viruses that have entered our body immediately look for a host cell and dock onto it. Red and white blood cells, liver cells, muscle cells, etc., often serve as host cells. The viruses release their own DNA/RNA into the host cell, which is forced to reproduce more viral particles instead of its own human cellular material. This viral material, produced by our own cells, can then be released into the body to infect more cells. The original host cell dies, and the viruses disperse throughout the body, docking onto new host cells [3].
CDC/ Alissa Eckert, MS; Dan Higgins, MAM, 2019-nCoV-CDC-23312, marked as public domain, Details on Wikimedia Commons
Viral infections can often trigger very harmless illnesses like a common cold. Our body and immune system usually handle this type of viral infection well. However, seasonal flu (influenza) is also a viral infection and can be quite dangerous for highly susceptible and weakened individuals [4]. Furthermore, there are certain viral diseases that can become chronic and drag on for years—such as HIV, liver inflammation (hepatitis), or herpes.
Due to their simple structure and lack of metabolism, viruses are immune to antibiotics [5]. Medicine has now developed virus-specific drugs (e.g., annual flu shots, HIV medications). However, it is rarely possible to completely defeat a virus with medication if our immune system cannot cope with it. This includes HIV drugs, which keep HIV under control for life but cannot completely destroy it.
Bacterial Infections
Unlike viruses, bacteria are single-celled microorganisms. They are independent, microscopic living organisms and can generally be fought with antibiotics.
Schematic Structure of a Bacterium:
Brudersohn, BakteriumSchema, CC BY-SA 3.0 DE
It is important to note that not EVERY bacterium makes us sick. Quite the opposite! Many bacteria are vital for human survival, such as for our intestinal flora or oral mucosa. Only about 1% of all bacteria cause disease in humans. However, if these penetrate our system, they can cause serious damage [6].
Similar to viruses, bacteria can be transmitted through droplets (respiratory tract and mucous membranes) and smear infections, but also through the urinary tract, the gastrointestinal tract, and via injuries to the skin. Some of the most common bacterial infections include pneumonia (pneumococci: a type of Streptococcus pneumoniae), meningitis (meningococci: Neisseria meningitidis), whooping cough (Bordetella pertussis), urinary tract infections, salmonellosis (Salmonella serotypes), and tuberculosis (Mycobacterium tuberculosis). In the case of injuries, bacteria can lead to wound and soft tissue infections [6].
In general, bacterial infections can be effectively treated with antibiotics. Antibiotics can prevent bacteria from multiplying or destroy them directly. This happens, for example, when the antibiotic prevents bacteria from producing a new cell wall during cell division. Without this protective shell, the bacterium bursts. Alternatively, protein biosynthesis is blocked, causing the bacterium to die.
In recent years, more antibiotic-resistant bacteria have been discovered—so-called multi-resistant, highly dangerous, and aggressive bacteria (e.g., MRSA). These pose a public health hazard and must be reported in Switzerland. A key reason for the development of resistance is, among other things, the frequent and unnecessary use of antibiotics—not only in medicine but also in livestock farming. The exchange of genetic material between bacteria can also lead to resistance [5].
These resistant germs are an increasing problem, especially in hospitals, and can lead to severe complications after surgery.
Parasites
Parasites include fleas, tapeworms, and head lice. Often, parasites are harmless and do not cause serious disease. For example, head lice bite into the scalp multiple times a day, causing annoying itching, but they can be easily eliminated with chemical treatments. Tapeworms can also often be easily treated with medication [7].
A hookworm:
DPDx Image Library, Hookworm larvaG, marked as public domain, details on Wikimedia Commons
In our region, the tick is probably the most dangerous parasite. With a tick bite, Lyme disease pathogens or the TBE virus can enter the blood and lead to inflammation of the meninges or the brain. However, not every tick is a carrier, and not every infected tick will infect us. The best way to protect yourself is to wear the right clothing when walking in the woods in spring and summer [2].
In addition to ticks, fleas are also disease vectors. In the 14th century, fleas were the primary cause of transmitting plague bacteria.
In tropical regions, the Anopheles mosquito is one of the most dangerous parasites, transmitting plasmodia during a bite, which causes malaria.
Apart from bites and stings, parasites (especially worms) are also transmitted through contaminated food, close contact with animals, or contaminated playground sand. In these cases, the worms enter our digestive tract through the mouth and develop into adult worms there. Regular and thorough handwashing can prevent worm infections.
Animals in general are often an intermediate stop for parasites. Fleas, worms, or insects find their way to us through dogs, cats, etc. That is why it is important for pet owners to deworm their pets regularly and prevent flea and tick infestations with the appropriate treatments [7].
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References
[1] https://www.gesundheitsforschung-bmbf.de/de/wie-keime-sich-ausbreiten-6432.php
[3] https://www.infektionsschutz.de/infektionskrankheiten/erregerarten/viren.html
[4] https://www.gesundheitsforschung-bmbf.de/de/infektionen-6299.php
[6] https://www.infektionsschutz.de/infektionskrankheiten/erregerarten/bakterien.html
[7] https://www.infektionsschutz.de/infektionskrankheiten/erregerarten/parasiten.html
Title Image Credits
National Institutes of Health (NIH), Human neutrophil ingesting MRSA, marked as public domain, details on Wikimedia Commons
