Fundamentals of Infection

Fundamental of Infection

• Infection is the invasion and multiplication of pathogenic microorganisms in tissue, leading to tissue injury and disease through cellular or toxic mechanisms.
• Disease is any deviation from a condition of good health and well-being.
• Infectious diseases are caused by the presence or growth of infectious microorganisms or parasites.
• Pathogenicity is the ability of a microbe to cause disease.
• Virulence is the degree of pathogenicity in a microorganism.

Principles of Infection

• Understanding the principles of infection is essential for healthcare workers.
• Infectious diseases are caused by pathogenic microorganisms like bacteria, viruses, parasites, or fungi.
• These diseases can spread directly or indirectly from one person to another (WHO).

Classification of Microorganisms

• Saprophytes are free-living microorganisms in soil and water that degrade organic matter.
• Parasites are microorganisms that enter and multiply in a host; they can be pathogens or commensals.
• Pathogens can cause disease.
• Primary pathogens cause disease in healthy humans.
• Opportunistic pathogens cause infection when host resistance is compromised.
• Facultative pathogens may cause infection when host resistance is lowered.
• Commensals live in harmony with the host without causing damage.

Classification and type of Infections

• Primary Infection is the initial infection.
• Reinfection is a subsequent infection by the same pathogen in the same host.
• Secondary/Opportunistic Infection is infection by a new pathogen in a host with lowered resistance due to a pre-existing infection.
• Focal infection is a localized infection.
• Co-infection/Mixed infection involves more than one pathogen.
• Nosocomial infections are hospital-acquired.
• Iatrogenic infection is physician-induced, acquired during medical processes.

Classification of Infections Based on Clinical Effect

• Inapparent/Subclinical infections have no apparent clinical effects.
• Atypical infections present with non-typical manifestations.
• Latent infections are latent or hidden.

Classification of Infections Based on Source

• Endogenous infections originate from the host's own body, also known as autoinfection.
• Exogenous infections come from an external source.

Source of Infection

• Humans can act as patients or carriers
  • Healthy carriers
  • Convalescent carriers
  • Temporary carriers
  • Contact carriers
  • Paradoxical carriers
• Animals.
• Insects act as vectors.
• Mechanical vectors.
• Biological vectors.
• Soil and water.
• Food.

Human and Animal Carriers

• Carriers are colonized with a pathogen but show no illness, and can transmit the pathogen to others.
• Passive carriers carry pathogens without illness.
• Incubatory carriers can transmit pathogens during the incubation period.
• Convalescent carriers are recovering individuals capable of transferring the pathogen.
• Active carriers have recovered but continue to harbor the pathogen.
• Zoonotic diseases are infections acquired from animal sources like rabies and Lyme disease.
• Arthropods can serve as reservoirs for infectious pathogens and act as vectors, including insects like mosquitoes, biting flies, and lice.
• Nonliving reservoirs include air, soil, dust, food, milk, and water.

Soil, Water and Food

• Soil can harbor microorganisms that survive for extended periods
• Fungi: Histoplasma capsulatum.
• Roundworm and Hookworm.
• Clostridium tetani.
• Water as a source of infections
• Contamination: V. cholera, HAV.
• Presence of aquatic vector: guinea worm.
• Pathogens in food can result from:
• External contamination, such as Staphylococcal food poisoning.
• Pre-existing infection in animal products, such as Brucellosis.

Mode of Transmission

• Five principal modes of transmission:
• Contact (Direct & Indirect)
• Airborne
• Droplet
• Vectors
• Reservoirs of infection are sources of microbes causing infectious diseases
• Can be living organisms or nonliving objects/materials
• Living reservoirs
• Humans
• Household pets
• Farm animals
• Wild animals
• Insects (mosquitoes, fleas, ticks, mites).

###Transmission Via Air (Inhalation)

• Organisms from the mouth and respiratory tract emerge during:
• Talking
• Coughing
• Sneezing (most dangerous, 10^5 to 10^6 organisms/sneeze)
• Infections transmitted this way include TB, Influenza, Measles, and Mumps.
• Size determines the fate of droplets:

  • 100µ fall to the ground and dry, pathogens remain viable causing infection from dust.

  • 100µ evaporate forming droplet nuclei (1-10µ) that remain airborne for 1-2 hours acting as a source of infection.