Infection (or infectious process) is the complex pathological process that has been evolved as the result of multiple interactions between the virulent bacteria and the susceptible host, followed by tissue damage, organ dysfunction and subsequent stimulation of immune response and other adaptive reactions.


Three main conditions are necessary for infectious process emergence.

  • First is the virulent causative agent presence, the second – pathogen’s ability to penetrate and invade the body; and third one is the host susceptibility to a certain pathogen.
  • The intensity of the infectious process is promoted by three above-mentioned conditions.
  • The first condition is based on causative agent virulence and its dose; the second depends on the efficacy of tissue defensive barriers, and the third – on immune and other adaptive system activities.
  • Place of pathogen adherence and penetration is known as a portal of entry for pathogenic bacteria.
  • According to their ability to cause infectious process, all of the microorganisms are divided into three main groups: obligate pathogenic, facultatively pathogenic and non-pathogenic or saprophytic microorganisms.

Obligate pathogenic microorganisms

  • It possesses highly aggressive virulence factors and in most cases induce infectious diseases as the result of initial susceptibility of the majority of human hosts (plague yersiniae, anthrax agents, tetanus and botulism clostridia, etc.)

Facultatively pathogenic microorganisms

  • It can trigger infectious diseases under the conditions of host defence insufficiency, e.g. in immunocompromised patients (opportunistic pathogens), and when inoculated in high doses (staphylo- and streptococci, pseudomonas, klebsiella and many other enteric bacteria, various fungi, etc.)

Non-pathogenic or saprophytic microbes

  • It usually doesn’t cause diseases. Many of them are normal habitats of the human body. They may play a role of “accidental” pathogens for humans, as they trigger some kind of infectious process only occasionally (i.e., with very low or negligible likelihood).