Principles of Disease and Epidemiology – Vocabulary Review
Pathology, Infection, and Disease
• Pathology – study of disease itself; seeks to understand structural and functional changes produced by disease.
• Etiology – the precise cause of a disease (infectious agent, genetic defect, toxin, etc.).
• Pathogenesis – mechanisms by which disease develops and progresses.
• Infection – invasion or colonisation of a host by pathogenic organisms.
• Infectious disease – any deviation from a healthy state that is produced by an infection.
Human Microbiome
• Establishes in utero – placental microbiome has Enterobacteriaceae & Propionibacterium.
• Birth canal – Lactobacilli transferred to neonate’s intestine during vaginal delivery.
• Additional colonisation from food, people, pets; communities fluctuate through life.
Types of Microbiota
• Normal (resident) microbiota – permanently inhabit the host; usually harmless under normal conditions.
• Transient microbiota – present days-months, then disappear.
Factors Shaping Microbiota
• Nutrient availability; temperature, pH, salinity, O2; mechanical forces (peristalsis, flow of saliva, urine); host defenses (lysozyme, IgA, complement).
• Host attributes: age, diet, nutritional state, geography, hygiene, housing, occupation, lifestyle.
• Crucial in immune-system maturation and modulation.
Symbiosis & Host Relationships
• Symbiosis – any close relationship between two organisms in which at least one depends on the other.
– Commensalism – one benefits, other unaffected (e.g. Staphylococcus epidermidis on skin).
– Mutualism – both benefit (e.g. synthesising vitamins K/B in colon while obtaining nutrients).
– Parasitism – one benefits at expense of host (influenza virus, helminths, protozoa, etc.).
• Opportunistic pathogens – normal/resident organisms that cause disease when ecological niche changes (immune suppression, altered site, etc.).
Microbial Antagonism (Competitive Exclusion)
• Normal flora suppress overgrowth of pathogens by:
– Competing for nutrients/space.
– Producing bacteriocins, acids, peroxides.
– Modifying local pH & O2.
• Classic examples: vaginal Lactobacillus lowers pH preventing Candida; gut flora outcompete Salmonella & C. difficile.
Koch’s Postulates
Same pathogen present in every case.
Pathogen isolated & grown in pure culture.
Culture must cause same disease in healthy, susceptible animal.
Pathogen re-isolated from inoculated animal & confirmed identical.
• Prove causation; foundation of medical microbiology.
• Exceptions – polymicrobial diseases, microbes only in humans (HIV), non-cultivable agents (Treponema pallidum, viruses), different pathogens with same syndrome (pneumonia).
Classifying Infectious Diseases
• Symptoms – subjective, felt by patient (pain, malaise).
• Signs – objective, measurable/observable (fever, rash, WBC count).
• Syndrome – specific constellation of signs + symptoms.
Communicability
• Communicable disease – spreads host-to-host (TB, measles).
• Contagious disease – easily & rapidly spread (influenza, chickenpox).
• Noncommunicable disease – not spread host-to-host (tetanus, diabetic ketoacidosis).
Occurrence Metrics
• Incidence – new cases/time period.
• Prevalence – total existing cases at given time (includes old + new).
Frequency Categories
• Sporadic – occasional (typhoid in US).
• Endemic – constantly present (common cold).
• Epidemic – many cases in short time (2014 Ebola in W. Africa).
• Pandemic – global epidemic (COVID-19, 1918 influenza).
Severity / Duration
• Acute – rapid onset, short duration (influenza).
• Chronic – slow onset, long duration (TB, hepatitis B).
• Subacute – intermediate (subacute sclerosing panencephalitis).
• Latent – dormant agent becomes active (shingles from varicella-zoster).
• Duration – avg. time from diagnosis to cure/death.
• Herd immunity – majority immune → limits spread; vaccination goal.
COVID-19 Severity Scale Example
• Asymptomatic, Mild, Moderate, Severe (pneumonia), Critical (SARS, ARDS, sepsis).
Extent of Host Involvement
• **Local