What are some examples of virulent diseases

Infectious diseases

Infectious diseases (colloquially: Infect) are diseases that are caused by pathogens. They can occur locally in a specific part of the body or extend over the entire body. Infectious diseases are transmitted to humans in different ways.

Short version:

  • Viruses, bacteria, fungi and other pathogens can be found everywhere in the environment. Many of them also colonize our body.
  • In most cases this is harmless to our health.
  • However, some pathogens can weaken our immune system in such a way that they make us sick.
  • Depending on the pathogen, the treatment of infectious diseases differs accordingly.

Information on this page:

How does an infectious disease develop?

An infectious disease, also called "infection" or "contagious disease", is a disease that is caused by certain pathogens. The disease breaks out when the pathogen penetrates the body and multiplies unhindered locally at the affected part of the body or - distributed via the bloodstream - throughout the body.

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Depending on the pathogen, the disease can develop acutely in a few days or over weeks, months, in rare cases even over years. In infectious diseases, the pathogen is in most cases a parasitic microorganism. Therefore, the sick person is also referred to as the host.

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The most common triggers of infectious diseases

The most common triggers of infectious diseases include:

  • Single-celled, seedless bacteriathat can lead to pneumonia, meningitis, Lyme disease, tuberculosis, cholera, and the plague
  • Single cells with a nucleus, so-called protozoa, which can trigger malaria, toxoplasmosis and tropical sleeping sickness
  • Pathogenic fungi (e.g. yeast or multicellular cells), through which fungal diseases such as aspergillosis and candidiasis can develop
  • Tissue animals like worms or arthropods (e.g. insects)
  • Virusesthat can trigger classic childhood diseases such as measles in children, mumps, rubella, diphtheria or chickenpox, as well as other infectious diseases such as rhinitis, flu, hepatitis and AIDS
  • Degenerate proteine, so-called prions, which can cause diseases such as Creutzfeldt-Jakob disease and possibly BSE

What types of infectious diseases are there and how are they transmitted?

In principle, a distinction is made between primary infections and secondary infections:

  • At Primary infections it is an initial infection in which the organism comes into contact with a pathogen for the first time.
  • At Secondary infections the already infected body is also attacked by another germ. So there is a double infection.
  • To a so-called Super infection it happens when a viral infection forms the basis for a further bacterial attack in the same organism.

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Differentiation from infectious diseases

Infectious diseases can also be divided into:

Type of transfer

  • Direct infection: In the case of a direct infection, the pathogen is transmitted from person to person without any intermediate step.
  • Indirect infection: In the case of an indirect infection, on the other hand, the pathogens are transmitted to humans via one or more carriers, such as insects, food or objects.

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Origin of the pathogen

Infections can also be differentiated according to the origin of the pathogen:

  • Endogenous infection (auto-infection): Here the pathogen that causes the disease comes from the body's own flora, e.g. from the intestinal or skin flora. In principle, these infections are harmless to a healthy organism. An infection usually only occurs when the immune system is weakened.
  • Exogenous infection: Exogenous infections are far more common. The pathogen penetrates the body from the outside or from the environment. Droplet infections, smear infections or an infection via insects are also exogenous infections.
  • Nosocomial infection: this is a special form that often occurs in hospitals, doctors' offices or other medical facilities. The pathogens can be transmitted here both endogenously and exogenously.

Path of transmission of the pathogen

Depending on how the pathogen is transmitted, the following important routes of infection can be distinguished:

  • Droplet infectione.g. by sneezing or coughing
  • Contact or smear infectione.g. through blood, mucus, feces, pus, breast milk, semen
  • Food infectione.g. from contaminated food
  • Infection over water
  • Vector infection by animal carriers such as ticks, mosquitoes or rodents

Entry gate of the pathogen

Depending on the pathogen's entry point, infectious diseases can be divided into:

  • Wound infection: The pathogens enter the organism through a wound.
  • Inhalation infection: The germs enter the body via the respiratory tract.
  • Inoculation infection: The infection takes place through a penetrating skin injury (e.g. insect bite or contaminated injection needle).
  • Percutaneous infection: The pathogens enter the body through the skin.
  • Permucous infection: The pathogen's portal of entry is the mucous membrane.

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A further distinction is made between:

  • Enteral infection: The pathogens enter the body through the intestines.
  • Fecal-oral infection: Feces or pathogens from the intestine enter the organism through the mouth (e.g. contaminated water).
  • Urogenital infection: The pathogens enter the body through the urinary tract.

Spread of infection

Infections can also be divided by their degree of extent. This includes:

  • Local infection: The pathogens only remain where they first attacked the body - at the entry point - without spreading further in other parts of the body.
  • Generalized infection: Here, the pathogens initially multiply at the point of entry (e.g. in the intestine) and pass through the bloodstream to the organs, such as the liver, skin, the lymphatic system or the nervous system.
  • Focal infection (focal infection): The pathogens (mostly bacteria) get from the source, which was initially caused by a local infection, via the blood to other parts of the body or organs, where they lead to an inflammatory or allergic reaction.
  • Systemic infection: This is an infection in which the pathogens get directly into the bloodstream and thus spread over entire organ systems (e.g. central nervous system) or in the entire organism.

How common are infectious diseases?

Viral diseases (e.g. hepatitis, AIDS, runny nose viruses, herpes) and bacterial infections (e.g. tuberculosis or diarrheal diseases) are the most common infectious diseases. Very often fungal, protozoal or worm infections also occur.

Some diseases only occur in certain regions of the world. For example, tropical diseases occur mainly in warmer areas. The possibility of spreading the disease vector (e.g. mosquito in malaria) also plays a role here, as the vectors usually multiply best in warm climates. Seasonal infectious diseases such as the flu (influenza) increase at certain times of the year.

What are the symptoms of infectious diseases?

The symptoms of an infectious disease can express themselves very differently. For example, skin infections often manifest themselves in the form of inflammation, which is accompanied by reddening, swelling, itching or pain.

Local heating of the affected area or fever (e.g. when malaria is transmitted) can also occur in the event of an infection with pathogens. Respiratory infections can in turn cause symptoms such as coughing, hoarseness (e.g. in the case of a laryngitis), runny nose or difficulty swallowing (e.g. in the case of tonsillitis).

Intestinal infections are also very common, causing symptoms such as diarrhea, abdominal cramps or pain. However, there are also infectious diseases that affect the skin or the urinary tract (e.g. urinary tract infection). Caries is also an infectious disease.

The symptoms that occur as a result of an infection and their intensity are linked on the one hand to the damaging effect of the pathogen itself - the so-called virulence - and on the other hand to the body's defenses.

An infection becomes an emergency when the pathogens multiply unhindered in the body and ultimately lead to blood poisoning (sepsis). Meningitis, renal pelvic inflammation or heart muscle inflammation can also be life-threatening for those affected if left untreated.

How are infectious diseases diagnosed?

Before starting treatment, it is determined which pathogen caused the infection. It is important to identify the pathogen and to know its properties:

  • Bacteria and fungi can also be determined with the help of a blood test or under a light microscope.
  • In the case of viruses, it is necessary to create a cell culture or an electron microscope.
  • Blood tests such as the so-called polymerase chain reaction in the laboratory are another molecular biological method to prove the genetic information of the pathogen.
  • Immunological laboratory tests that detect antibodies in the blood can also be used to diagnose a pathogen.

How are infectious diseases treated?

The correct treatment of infectious diseases basically depends on the diagnosis. Depending on the clinical picture and pathogen, antibiotics are used for bacterial infectious diseases or antivirals for viral diseases. In life-threatening situations (e.g. with sepsis) there is usually not enough time to diagnose the exact pathogen causing the disease. Therefore, antibiotic treatment or antimycotic therapy (for fungi) is often started immediately.

In some cases it is necessary to treat patients with highly contagious pathogens in special isolation wards in order to reduce the risk of infection. In principle, however, it is also possible to isolate infected patients in conventional hospital rooms. It is important, however, that in this case the patient's visitors adhere to the relevant hygiene guidelines (e.g. hand disinfection) so as not to endanger themselves or other patients.

Serious infections also occasionally require surgical treatment. With some viruses (e.g. hepatitis A and B, influenza, TBE) it is also useful to get vaccinated against the pathogen.

How can you protect yourself from infectious diseases?

To protect yourself and others, it is important to know how pathogens are transmitted and how they enter the body. Basically, sufficient hygiene and a healthy lifestyle should be ensured. To do this, you should observe the following protective measures:

  • Wash your hands regularly, especially after using the toilet, before and after eating, after preparing food, after contact with infected objects, wounds or people and after cleaning your nose.
  • Regularly clean your sanitary facilities and your kitchen.
  • Keep your hands away from your face as far as possible, as possible pathogens on the hands, which are transmitted, for example, by shaking hands, can quickly enter the body if they come into contact with the eyes, nose or mouth.
  • Take off your shoes before entering your living space. This will prevent the pathogens from spreading in your home.
  • To reduce the number of viruses in closed rooms, you should ventilate regularly.
  • Try to avoid places where there are many people (e.g. subway) when you have a cold spell.
  • Use condoms during sexual contact.
  • Drink enough fluids to keep your mucous membranes moist. This will reduce the risk of pathogens entering your body through your mucous membranes (e.g. nose, mouth or eyes).
  • Strengthen your immune system by exercising regularly and eating a balanced diet.

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Tanja Unterberger, Bakk. phil.
Medical review:
Univ.-Prof. Dr. med. Florian Thalhammer
Editorial editing:
Silke Brenner

Status of medical information:

Suttorp et al. (2003): Infectious Diseases: Understanding, Recognizing, Treating. Thieme Verlag, Stuttgart.

Suerbaum et al. (2012): Medical Microbiology and Infectiology. Springer Verlag, Berlin Heidelberg.

Bauernfeind & Shah (1990): Lexicon of microbiology and infectiology. Schattauer Verlag, Stuttgart.

PROHYG 2.0 (2011): Organization and strategy of hospital hygiene. 2nd edition, Federal Ministry of Health (BMG), Vienna. (available at: https://www.bmgf.gv.at/home/Service/Broschueren)

S2k guideline 055/006: Therapy and prophylaxis of opportunistic infections in adult HIV-infected patients (status: 01/2015, valid until 12/2018)
http://www.awmf.org/uploads/tx_szleitlinien/055-006l_S2k_Therapie_Prophylaxe_opportunistische_Infktionen_HIV_infIERT_erwachsene_Patienten_2015-01_verlaengert.pdf (online, last: 31.08.2017)

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So-called waterborne diseases are diseases that are transmitted through the water.

Droplet infection

A droplet infection is the transmission of pathogens through tiny droplets of secretion through the air.

Routes of infection

Pathogens can get into the body in different ways, most of them have a typical route of transmission.