Infection with pathogenic microorganisms can occur only through the damaged skin and mucous membranes of the eye, respiratory, digestive, and genitourinary tracts. Infection through intact skin is extremely rare, as the skin is difficult to permeate for most microorganisms. However, even the most insignificant damage to it (insect bite, needle prick, microtrauma, etc.) can lead to infection. The place of penetration of the pathogen into the human or animal body is called the entrance gate of the infection. If they are mucous membranes, three types of infection are possible:
- propagation of the pathogen on the surface of epithelial cells
- penetration into cells with subsequent intracellular reproduction
- penetration of the pathogen through the cells and its spread throughout the body.
Infection of a person occurs in one of the following ways:
- Airborne dust
- Fecal-oral. The causative agent is excreted with feces or urine, infection occurs through the mouth with the use of infected foods or water
- Transmissible, that is, through the bites of blood-sucking arthropods
- Contact – direct contact with the patient, convalescent, bacterium or through contaminated household items, i.e., indirect contact
- When using non-sterile medical devices, especially syringes.
- Vertical, i.e., from the mother to the child through the placenta, during childbirth or immediately after.
Each of the pathogens of infection has a predisposition to develop in certain organs, tissues, and the typical signs of the disease depend on it. Also, the infectious dose of pathogens (the minimum number of microbes that can cause human disease) is of great importance for infection.
After entering the blood or tissue of the body, bacteria do not immediately activate. They first need to go through the growth stage – the incubation period – which, depending on the type of bacteria, can last from a few hours to a year or even more.
They cause illnesses either by damaging tissues, as occurs in typhoid, meningitis, and pertussis or through the release of potentially toxic substances called toxins. Toxins lead to food poisoning and diseases such as tetanus. In the case of food poisoning, these substances cause irritation of the intestines, vomiting, and diarrhea, while in tetanus and diphtheria they cause serious damage to the nervous system.
The body responds to the infection by mobilizing its defenses. The first sign is inflammation, the presence of which indicates a more intensive blood supply to the affected area. White blood cells attack and surround uninvited guests in the area of direct infection. At the same time, the body produces so-called antibodies that can affect the interventionists. However, the amount they produce is not enough to operate for several days.
Even after the fading of the infection, some of the remaining bacteria may still remain potentially dangerous. People, in the organism of which such bacteria live, are called carriers.