How is bacterial infection diagnosed

An infection is the penetration, retention and transfer of pathogens (bacteria, viruses, fungi, etc.) into a body, with subsequent symptoms of illness. Virus infections and bacterial infections are the most common, but fungal infections, infections by protozoa (unicellular organisms) or worm infections also occur.

The infection must be differentiated from mere colonization by bacteria and certain fungi that live on the skin and mucous membranes without invading. If the skin or mucous membrane is damaged or if the immune system is weak, these germs can also cause an infection (endogenous infection), but remain relatively harmless.

Humans need microorganisms to break down food in the intestines, just as some of these, on the other hand, need humans to feed and reproduce. In addition, this colonization flora keeps away dangerous germs by occupying the skin and the mucous membranes. In most cases this symbiosis remains in equilibrium, which does not harm the human organism. But there are also germs that break out of this equilibrium and harm people, even threaten their lives.

Development of infections
Infections occur when germs ("pathogens") such as bacteria, viruses or other microorganisms penetrate the body and cause damage. Whether the germs multiply and how severe the infection is depends on the relationship between the germ (“guest”) and the person (“host”). Most of the time, a certain amount of pathogen is necessary for an infection. If only individual pathogens get into the body, this does not lead to an infection. Well-adapted pathogens themselves ensure that they do not harm their host too much.

Nosocomial infections are those that are acquired in the hospital. They are different from other, outpatient infections. Nosocomial infections are particularly common due to urinary catheters or artificial ventilation. So-called problem germs such as pseudomonads, which are particularly difficult to treat with antibiotics (MRSA), are often involved.

Routes of infection
In the case of external transmission (exogenous infection), there are the following infection routes:

  • Through the air (droplet infection),
  • Smear infection,
  • sexually,
  • over wounds (e.g. needle stick injury, needle stick injury),
  • intrauterine,
  • via a vector (mosquitoes, ticks, bed bugs),
  • from body fluids (at birth).

The symptoms of an infectious disease are related on the one hand to the harmful effects of the pathogen, but on the other hand also to the reaction of the immune system. Correspondingly, infections in people with a weak immune system are often dangerously inconspicuous and difficult to diagnose because fever, malaise and inflammation parameters in the blood and infiltrates in the lungs are absent.

Mute or manifest
A distinction is made between one, depending on the immunity and resistance of the infected organism

  1. silent infection that does not cause the disease to break out,
  2. abortive infection, with mild symptoms, as well
  3. manifest infection, with a clear outbreak of the infectious disease.

Temporal course
According to the time sequence, one differentiates:

  1. foudroyant, peracute, lightning fast, very dangerous,
  2. acute, beginning suddenly, severe effects,
  3. chronic, beginning gradually, extending longer,
  4. recurrent, repetitive,
  5. latent, remaining hidden for long periods of time,
  6. subacute, less violent than acute.

In addition, one differentiates

  1. Local infections,
  2. cyclical general infections,
  3. sepsis (blood poisoning)
  4. or intoxications.

An important prerequisite for combating an infectious disease is an accurate diagnosis; H. recognizing the pathogen and its properties. In cases with a threatening course of the disease, however, an exact diagnosis of a serious infectious disease cannot be waited for, but chemotherapy (e.g. with antibiotics or antimycotics) is started, which should target all probable pathogens (calculated therapy).


  1. Bacteria detection in the light microscope after staining (e.g. Gram staining), in the case of viruses in the electron microscope,
  2. Antibody detection,
  3. Animal experiments,
  4. Detection of characteristic fragments of the pathogen,
  5. Skin tests with weakened pathogens.

Through hygiene, disinfection, vaccinations, antibiotics and the use of other chemotherapeutic agents, people try to protect themselves from pathogens.