General

Viruses, bacteria, parasites, and their counterparts

January 28, 2021

Pathogenic Pathogens and Germs

The coronavirus (COVID-19) has been a global concern for over a year now, affecting the whole world as well as us here in Switzerland. As the name suggests, SARS-CoV-2 falls under viral infections. But what exactly are viruses? And what is the difference between bacterial infections and parasites? How are these treated? What is effective?

Transmission and Infection

An infection followed by a subsequent disease of any kind can occur in various ways. The most well-known and widespread is through droplet infection: as we breathe and speak, and especially when coughing or sneezing, viruses and bacteria attach themselves to tiny droplets released into the air (aerosols) and are inhaled by other people. In this way, these pathogens can be transmitted from person to person.

However, smear and contact infections also pose a significant risk. In this case, pathogens are left on surfaces (doorknobs, handrails in public transport, etc.), and countless other people can become infected through contact with these objects [1].

Further transmission can occur through bodily fluids (blood, saliva, sexual fluids, etc.), allowing for the transmission of various diseases during sexual intercourse, such as the bacterial pathogens of syphilis and gonorrhea or viral pathogens like herpes, hepatitis B, and HIV [1].

Additionally, food and drinking water, especially in poorer countries, remain sources of infections. For instance, in food, Salmonella (bacteria) or hepatitis A viruses can lead to diarrhea. The most well-known example of drinking water as an infection source is cholera (bacterium).

While some pathogens can only be transmitted in specific ways, others can be spread in multiple ways. A prime example is E. coli bacteria. They can be transmitted through contaminated food (especially meat), contaminated water, and even directly from person to person [1].

Some infectious diseases are only spread from person to person (e.g., measles). Diseases transmitted from animals to humans are called zoonoses [1]. The most common animal carriers are insects. The Anopheles mosquito, the primary host of malaria (parasite), and the yellow fever mosquito, as well as the Asian tiger mosquito (which transmits dengue fever and other life-threatening tropical diseases), are some of the most well-known sources of zoonoses. Even in our latitudes, insects can be potential carriers of disease: ticks can transmit early summer meningoencephalitis (FSME, viral infection), a partly life-threatening meningitis, or Lyme disease (bacterial infection) [2].

Besides insects, birds are the second most important carriers of zoonoses (bird flu).

Getting Infected Doesn’t Mean Getting Sick

Regardless of whether a virus or bacterium invades the body, the immune system responds with a complex and sophisticated defense reaction: foreign cells are attacked.

If the body can resolve the situation relatively easily and quickly without much commotion, you do not get sick and hardly notice anything.

Important: under certain circumstances, you may still be contagious, as you carry the disease even though you don't fall ill!

If the pathogen load is too large or too complex for the body's defense mechanism (it is not fast enough) or if the body detects them too late, the pathogens can spread within the body and cause an infection: you fall ill and experience symptoms. Fever promotes the defense reaction, and once the pathogens are under control and reduced, you feel better and recover.

It is impossible to say precisely why and when one becomes ill: not everyone falls sick! This highly depends on the specific and individual immune system. And also on the number of pathogens (germ load) and the danger posed by the pathogen.

Viral Infections

US government, Infectious bronchitis virus, marked as public domain, details on Wikimedia Commons

Viruses are by far the most important pathogens for infections of the upper respiratory tract and diarrheal diseases.

Viruses are very simple particles composed only of a strand of genetic information (DNA/RNA) and some protein cells. Thus, viruses have no cells or metabolism of their own, so they are not independent living organisms.

Schematic Structure of a Virus:

Y_tambe, Virion, CC BY-SA 3.0

Viruses invade animal, plant, and human organisms and seek a host cell to reproduce. However, certain viruses can also survive on surfaces for a while. If they do not find a host cell for reproduction, they can eventually die over time. It's essential to note that not all viruses that infect humans make us sick. Our immune system often reacts very quickly and successfully combats the pathogens [3].

Once a pathogen enters our body, we are infected and can transmit the virus to others - EVEN WITHOUT SHOWING SYMPTOMS OR FEELING SICK! This usually occurs through close direct contact, especially in enclosed spaces, via coughing, sneezing, or shaking hands. Yet, indirect contact, such as on doorknobs, handrails on buses/trams/trains, can also contribute, as mentioned, since viruses can survive on surfaces for some time.

Viruses that enter our bodies immediately seek a host cell and attach themselves there. Host cells often include red and white blood cells, liver cells, muscle cells, etc. Viruses release their own DNA/RNA in the host cell, forcing it to reproduce more virus particles instead of its own, human cell material. This virus material produced by our own cells can be released into the body, infecting more cells. The original host cell dies, and the viruses spread throughout the body, attaching themselves to 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 our immune system can usually handle this type of viral infection quite well. But seasonal flu (influenza) is also a viral infection and can become very dangerous for particularly susceptible and weakened individuals [4]. Furthermore, there are specific viral diseases that can become chronic over the years – HIV, liver inflammation (hepatitis), or herpes.

Due to their simple structure and lack of metabolism, viruses are immune to antibiotics [5]. In medicine, virus-specific medications have now been developed (e.g. annual flu vaccine, HIV medications), but it is rarely possible to completely overcome a virus with which our immune system cannot cope solely with medication. These include HIV medications, which keep the HIV under control for life but cannot completely destroy it.

Bacterial Infections

In contrast to viruses, bacteria belong to unicellular microorganisms. As such, they are tiny living organisms in their own right and can generally be combated with antibiotics.

Schematic Structure of a Bacterium:

Brudersohn, Bacterium Schema, CC BY-SA 3.0 DE

It's important to note that not EVERY bacterium makes us sick. On the contrary, many bacteria are vital for humans, such as those in our gut flora or mouth mucosa. Only about 1% of all bacteria cause disease in humans. However, if they invade our system, they can cause severe damage [6].

Bacteria can be transmitted similarly to viruses through droplet (respiratory and mucosa) and smear infections, but also via the urinary tract, gastrointestinal tract, and through skin injuries. 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 injuries, bacteria can lead to wound and soft tissue infections [6].

Generally, bacterial infections can be effectively treated with antibiotics. Antibiotics can prevent bacteria from multiplying or destroy them directly. This occurs, for example, by preventing bacteria from producing a new cell wall during cell division due to the antibiotic. Lacking this protective outer layer, the bacterium bursts. Alternatively, protein biosynthesis is blocked, causing the bacterium to die.

In recent years, more antibiotic-resistant bacteria have been discovered, known as multi-resistant, very dangerous, and aggressive bacteria (e.g., MRSA). These pose a threat to public health and must be reported in Switzerland. A major reason for the development of resistance is, among others, the frequent unnecessary use of antibiotics - not only in medicine but also in animal husbandry. Moreover, the exchange of genetic material between bacteria can lead to resistance [5].

Especially in hospitals, these resistant germs are becoming an increasing problem and can lead to severe complications after surgeries.

Parasites

Parasites include fleas, tapeworms, and head lice, among others. Often, parasites are harmless and do not cause diseases. For example, head lice bite into the scalp several times a day, causing an annoying itch, but they can be easily combated with chemical agents. 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 latitudes, ticks are probably the most dangerous parasite. A tick bite can introduce Lyme disease bacteria or FSME virus into the blood, leading to inflammation of the meninges or brain. However, not every tick is a carrier, and not every infected tick will necessarily infect us. The best prevention is wearing appropriate clothing when walking in the woods during spring and summer [2].

In addition to ticks, fleas are also carriers of disease. In the 14th century, fleas were the main cause of the spread of plague bacteria.

In tropical regions, the Anopheles mosquito is one of the most dangerous parasites, transmitting Plasmodium, which causes malaria, for instance.

Besides bites and stings, parasites (especially worms) are also transmitted through infested food, intense contact with animals, or contaminated playground sand. The worms enter our digestive tract through the mouth and develop into mature worms. Regular and thorough handwashing can prevent worm infestation.

Animals, in general, are often intermediate hosts for parasites. Fleas, worms, or insects find their way to us through dogs, cats, etc. Therefore, it is crucial for pet owners to regularly deworm their pets and use suitable products to prevent flea and tick infestations [7].

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