Infectious process and infectious disease - etiology, epidemiology, chain of infection, clinical manifestations of infections
Infectious process: Sequence of events involving interaction between a pathogenic microorganism and the host, leading to colonization, invasion, and possible tissue damage.
Infectious disease: Clinically manifested consequence of the infectious process, associated with local or systemic signs and symptoms.
Etiology
Causative agents (pathogens)
Bacteria: e.g., Staphylococcus aureus, Mycobacterium tuberculosis.
Viruses: e.g., Influenza virus, HIV.
Fungi: e.g., Candida albicans, Aspergillus.
Parasites: e.g., Plasmodium (malaria), Entamoeba histolytica.
Prions: e.g., Creutzfeldt-Jakob disease.
Epidemiology
Study of distribution and determinants of infections in populations.
Key terms
Reservoir: Natural habitat of the pathogen (humans, animals, environment).
Source of infection: Immediate origin of infection (another person, contaminated food).
Mode of transmission: How the pathogen spreads.
Chain of Infection
The chain of infection describes how an infectious agent spreads from its source to a susceptible host.
- 1 Infectious Agent
The pathogen (microorganism) that causes disease.
Bacteria: Streptococcus pyogenes (causes pharyngitis).
Viruses: Influenza virus (causes flu).
Fungi: Candida albicans (causes thrush).
- 2 Reservoir
The natural habitat where the pathogen lives, grows, and multiplies.
Humans: carriers of hepatitis B, asymptomatic COVID- 19 patients.
Animals: bats (reservoir for rabies virus), rodents (plague).
Environment: soil (reservoir for Clostridium tetani ), stagnant water (Legionella).
- 3 Portal of Exit
The route by which the pathogen leaves the reservoir.
Respiratory tract: coughing or sneezing (tuberculosis, influenza).
Gastrointestinal tract: feces or vomit (cholera, hepatitis A).
Urogenital tract: urine, semen, vaginal secretions ( gonorrhea, HIV).
- 4 Mode of Transmission
The way the pathogen is transferred to a new host.
Direct contact: person- to- person (herpes simplex via kissing).
Droplet: large droplets from cough or sneeze (COVID- 19, influenza).
Indirect contact (fomites): shared objects (MRSA from towels, door handles).
Vector-borne: insects (malaria by mosquitoes, plague by fleas).
- 5 Portal of Entry
How the pathogen enters a new host.
Respiratory tract: inhaling droplets (influenza).
Gastrointestinal tract: ingesting contaminated food (typhoid fever).
Genitourinary tract: sexual transmission (syphilis, HIV).
Skin: cuts, puncture wounds (tetanus).
- 6 Susceptible Host
A person who lacks immunity or resistance to the pathogen.
Immunodeficiency (HIV infection, chemotherapy patients).
Chronic diseases (diabetes, cancer).
Age extremes (infants, elderly).
Malnutrition.
Unvaccinated individuals.