Introduction to Infectious Diseases: Terms and Concepts
Core Terminology and Fundamental Concepts
Etiology: The study of the cause of disease.
Etiological Agent: The pathogen associated with a particular disease.
Ecological Niche: The habitat in which a species lives.
Infection: The multiplication of organisms in an individual host or a population.
Disease: A disorder of structure or function in the host that adversely impacts the host and is not simply a result of physical injury.
Infectious Diseases: Diseases caused by pathogenic microorganisms, including:
Bacteria
Viruses
Parasites
Fungi
Prions
Communicable Nature: Some infectious diseases are communicable, meaning they can be spread directly or indirectly from one animal to another.
The Microbiome and Colonization Resistance
Microbiome: A microbiota built by a diverse collection of microorganisms covering many animal tissues, comprising:
Fungi
Bacteria
Archaea
Viruses
Mites
Commensal Microorganisms: Fundamental for maintaining a healthy microbiome by preventing the colonization of harmful pathogens.
Mechanisms of Colonization Resistance: Essential for shaping and maintaining the skin microbiota.
Disease Classification and Reporting
Communicable Disease: Any infectious disease that spreads from individual to individual. It may require a specific mode of transmission, such as:
Vector transmission
Sexual transmission
Contagious Disease: Infectious diseases that spread easily from an individual to another individual (by air or physical touch).
Notifiable Disease: Legally mandated to be reported to public health authorities for the purpose of controlling and/or preventing the spread of the disease. This enables:
Surveillance
Outbreak detection
Public health response
Reportable Disease: A broader term that refers to diseases that must be reported to relevant authorities for tracking and monitoring. They are state-regulated.
Jurisdictional Rule of Thumb:
State-reportable
Nationally-notifiable
Non-Infectious Reportable/Notifiable Conditions: Not all notifiable/reportable diseases are infectious diseases. Examples include:
Cancer
Lead poisoning
Congenital conditions
Microbe-Host Relationships and Pathogen Categories
Free-Living Organism: An organism whose ecological niche is outside the animal body.
Commensal Organism: A microorganism that is a normal inhabitant of the animal body.
In commensal relationships, either the microbe or host (or both) derives benefit; neither is harmed under normal circumstances.
Pathogen: A microorganism capable of causing disease. Includes commensals and non-commensals (obligate pathogens).
Facultative Pathogen: Microorganism which can infect and multiply in hosts, occasionally causing disease, but also capable of multiplying in the environment.
A microorganism that can live outside the host (free-living or commensal) but can cause disease when it enters the host.
Opportunistic Pathogen: A microorganism that causes disease only when the host's defenses are compromised (e.g., immunosuppression, wounds, microbiota disruption).
Originate from commensal or free-living organisms; distinguished by origin as:
Commensal Pathogens: Originating from internal normal flora.
Environmental Pathogens: Originating from environmental sources.
Obligate Pathogen: A microorganism that must produce disease to transmit and thereby survive evolutionarily.
Definition of Disease in this Context: Damage to the organism at a molecular level, with or without clinical signs.
Obligate pathogens are not commensals, but they may produce subclinical infections (no evident clinical signs).
Zoonotic Pathogen: A microorganism that is a commensal or pathogen on animals, but can be transmitted to humans and cause disease.
Transmission occurs through:
Vectors (e.g., ticks)
Direct contact with the animal or its products
Pathogen Carriers of Infectious Disease
Incubatory Carrier: Host that is incubating the pathogen but not yet ill (may involve shedding of pathogen).
Convalescent Carrier: Host that is recovering (exhibits no clinical signs), but continues to harbor and may potentially transmit the pathogen to others.
Active (Persistent) Carrier: The animal recovered from the disease, but could not eradicate the pathogen and continuously sheds it to the environment.
Latent (Persistent) Carrier: The pathogen is harbored internally and is typically shed only intermittently—often triggered by stress or immunosuppression.
Subclinical Carrier: Animal that is infected with a pathogen but does not exhibit clinical signs of disease. Despite appearing healthy, the carrier can still shed the pathogen and contribute to its transmission.
Clinical Carrier: Infectious animal with a clinical infection in a peracute, acute, or chronic phase.
Koch's Postulates and Their Limitations
Koch's Postulates:
The suspected pathogen must be found in every case of disease and not be found in healthy individuals.
The suspected pathogen can be isolated and grown in pure culture.
A healthy individual subject to infection with the suspected pathogen must develop the same signs and symptoms of disease.
The pathogen must be re-isolated from the new host.
Limitations of Koch's Postulates:
Inability to isolate the organism in pure culture or in cell-free media.
Organisms for which a single species acts as host (commensal – opportunistic pathogen).
Subclinical infection.
Organisms that cause 'distant' disease.
Disease caused by toxins.
Diseases that require coinfection.
Organisms associated with tumors.
Five Stages of an Infectious Disease
1. Incubation Period: Pathogen multiplies without clinical signs.
2. Prodromal: General and unspecific signs.
3. Illness: Signs and symptoms are more severe and specific.
4. Decline: Number of pathogens and clinical signs begin to decline.
5. Convalescence: Return to healthy state.