Various Forms of Infection

Various Forms of Infection

  • According to the origin of infection, there are exogenous and endogenous infections. Exogenous infection appears from the external source of infection, while endogenous evolves after activation of internal infection.
Various Forms of Infection
  • Localization and the capacity of spreading determine local or generalized forms of infection. In the latter case, the agent spreads from the initial site throughout the whole body.
  • If the bacteria enter the bloodstream, it may stay there for some time. This state is known as bacteremia (in case of viral diseases – as viremia). Transitory bacteremia occurs in enteric fever, rickettsioses, tularemia and other diseases.
  • In the case of immune system dysfunction and infection, severity bacteremia is followed by further microbial dissemination. This complication is known as sepsis or septicemia, where the bacteria are able to propagate within the blood and tissues.
  • According to current definitions, sepsis is designated as life-threatening organ dysfunction caused by a dysregulated host response to infection.
  • The infection might be documented or clinically suspected.
  • Pathogenesis of sepsis is based on abnormal hyper- or hypoactivity of the immune response against systemic infection coupled with enhanced virulence of microbial pathogens. Immune hyperactivity is followed by the massive release of proinflammatory cytokines (“cytokine storm”). This leads to endothelial damage, deep microcirculation disturbances and intensive procoagulant activity that ensure malfunction of inner organs.
  • And vice versa, highly virulent microbial pathogen causes similar body injuries on the background of the inefficient immune response.
  • If not treated perfectly, sepsis progresses into septic shock with multiple organ failure and tissue damage.
  • In addition, numerous bacteria are able to produce exotoxins (causative agents of diphtheria, tetanus, gas gangrene etc), which spread with blood flow. This state is named toxemia.

Depending on the duration of their course the infections are divided into acute, protracted and chronic.

Acute infections

  • These are followed by the sudden onset and a comparatively short time of their course (usually less than 1 month).
  • Among them are influenza, acute respiratory viral infections, measles, typhoid fever and many others.
  • The diseases with protracted or chronic (elapsing more than 3-6 months) courses are lingering (HIV, tuberculosis, syphilis, brucellosis etc).

According to their manifestations, typical, atypical, abortive, latent and inapparent infections can be outlined.

  1. Typical infection demonstrates all the symptoms of the disease; atypical forms are characterized by some unusual infection course.
  2. Abortive infection undergoes abrupt interruption after typical disease onset. It usually occurs due to the rapid activation of the intensive immune response or by the administration of highly efficient specific treatment.
  3. Latent infections: A large number of infections may be hidden or without obvious clinical manifestations (latent infections). Nevertheless, the microbial pathogen remains slowly propagating here, and the infection can undergo transformation into the typical form under different internal or external stimuli.
  4. Inapparent: Asymptomatic form of infection was named inapparent, where the clinical symptoms of the disease are not determined usually because of efficient control of the infection by the immune system.

Persistence is the special state of infectious agent preservation in the host. The activity of pathogen is minimal, and it may stay in the human body for a long period of time, maintaining the infection.

Mixed infection means the combined infectious process caused by more than one microbial species. The opposite case is named mono-infection.

Secondary infection

  • Sometimes primary infectious agent suppresses greatly the local or general immune reactions or impairs the metabolism of the host. In that case, the organism becomes susceptible to other microbial pathogens. This is known as a secondary infection.
  • Such an example here is the development of bacterial pneumonia after the primary influenza attack.


  • It is a repeated infection caused by the same microbial species after the complete recovering from the previous case of the same infection.
  • It is possible when sufficient anti-infectious immunity is not formed (gonorrhoea, helicobacteriosis, and other infections).

Relapse is the return of symptoms of the infectious disease, which ensues from the incomplete recovery of the patient (in case of relapsing fever, Brill-Zinsser disease, etc.)

Superinfection means the additional infection of the host with the same microbial species, where the previous infection has not ended yet. For instance, superinfection occurs in syphilis – the patient in the period of tertiary syphilis can be superinfected with T. pallidum again with the development of secondary syphilis symptoms.