Forms of Infection

Classification by origin and etiology

Depending on the source of the pathogen, two main forms are distinguished:

• Endogenous infections — develop due to microflora permanently residing in the body. The process is triggered by a decline in immune defense or a sudden increase in the virulence of the microbe itself.
• Exogenous infections — occur when a pathogen enters from the external environment through specific portals of entry.

Based on the nature of the causative agent, diseases are divided into bacterial (diphtheria, tuberculosis, dysentery), viral (influenza, rabies, HIV), fungal (candidiasis, trichophytosis), protozoan (malaria, giardiasis), and prion (kuru, scrapie). If the disease is caused by a single microorganism, it is a monoinfection; if by several, it is a mixed infection (coinfection or polyinfection).

Course variants and special forms

Infections may present with prominent clinical features (manifest) or subclinically (asymptomatic). By duration, acute, subacute, and chronic processes are distinguished. At the same time, some diseases occur only in an acute form (measles, scarlet fever, plague, smallpox), whereas others are capable of taking any of the three forms (hepatitis B, brucellosis, dysentery).

Special attention should be paid to specific variants of interaction between the macro- and microorganism:

• Carriage — the process proceeds without clinical symptoms.
• Latent form — prolonged occult coexistence. The pathogen hides inside cells (for example, herpesviruses can persist in a genome-integrated form) and reactivates only under unfavorable conditions.
• Secondary infection — the addition of a new pathogen to an existing one (e.g., bacterial pneumonia following influenza).
• Reinfection — repeated infection with the same microbe after complete recovery.
• Superinfection — a new infection with the same causative agent while the current disease has not yet resolved.
• Relapse — the return of symptoms due to the activation of the pathogen remaining in the body after the illness (a classic example is Brill-Zinsser disease as a relapse of epidemic typhus).

Degree of tissue spread

By the extent of bodily involvement, infections are divided into local (focal), when the microbe remains in a single organ (furuncle, tonsillitis), and generalized. The latter have several scenarios of development:

1. Bacteremia or viremia — the pathogen simply circulates in the blood without multiplying in it, in order to reach target organs (typhoid fever, paratyphoid fevers, salmonellosis).
2. Pyemia (septicopyemia) — bacteria travel via the bloodstream, settle in tissues, and form secondary purulent foci. They do not multiply in the blood itself and perish.
3. Septicemia (sepsis) — microbes not only circulate but also actively multiply directly in the bloodstream (characteristic of streptococci, staphylococci, Pseudomonas aeruginosa, and certain strains of E. coli).
4. Septicopyemia — the most severe form, combining the multiplication of pyogenic bacteria in the blood with the formation of metastatic purulent foci throughout the body.
5. Toxinemia — the microbe itself resides at the site of entry and multiplies there, but releases exotoxins into the blood, which cause the systemic disease (tetanus, diphtheria).

Features of infections in immunodeficiencies

Against the background of pronounced immunosuppression (e.g., in HIV), opportunistic infections (OIs) develop. Their main feature is an atypical course. They develop gradually, affect atypical organs, easily generalize, and frequently occur as mixed infections, demonstrating high resistance to treatment.

Diagnosis of such forms is severely hindered. Standard serological tests are often ineffective because the depleted immune system does not produce antibodies. Microbiological cultures, PCR, instrumental methods (CT, endoscopy), and ex juvantibus diagnostics—confirming the diagnosis via a positive response to trial therapy—come to the forefront.

Clinical markers for suspected HIV

Initial HIV diagnosis requires exceptional precision and a comprehensive approach (evaluation of clinical, epidemiological, and laboratory criteria), as an error leads to severe psychological trauma (AIDS phobia, suicide attempts, depression). Infection should be suspected if, following a mononucleosis-like or influenza-like episode, the patient develops:

• Fever or polylymphadenopathy lasting more than 1 month.
• Unexplained loss of more than 10% of body weight.
• Prolonged, recurrent pneumonias unresponsive to standard treatment.
• Severe pyogenic-bacterial, parasitic infections, or sepsis.
• Subacute encephalitis with the development of dementia in a previously healthy individual.
• Persistent pyodermas or hairy leukoplakia on the mucous membranes.
• Chronic inflammations of the reproductive system of unclear etiology.

Class and exam questions

What else to study on this topic

• Management tactics for severe drug-induced skin reactions (Stevens-Johnson syndrome, toxic epidermal necrolysis) in HIV.
• Pathogenesis of prion infections: from kuru to scrapie.
• Mechanisms of formation of secondary purulent foci in pyemia and septicopyemia.
• The role of exotoxins in the development of toxinemia, using diphtheria and tetanus as examples.
• Features of microbiological diagnosis of mixed infections.

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.