Principles of Disease, Epidemiology, and Antimicrobial Therapy

Principles of Disease and Epidemiology

Key Public Health Terms

• Incidence: The number of new cases of a disease in a population during a specific time period.

• Prevalence: The total number of cases (new and existing) of a disease in a population at a given time.

• Sporadic: Diseases that occur only occasionally and at irregular intervals.

• Endemic: Diseases constantly present in a population or geographic area.

• Epidemic: A sudden increase in the number of disease cases above what is normally expected.

• Pandemic: An epidemic that spreads across countries or continents, affecting a large number of people.

• Herd Immunity: Resistance to the spread of a contagious disease within a population due to a high proportion of individuals being immune.

Types of Disease

• Acute: Rapid onset, short duration.

• Chronic: Slow onset, long duration.

• Sub-acute: Intermediate between acute and chronic.

• Latent: Disease with periods of no symptoms, but pathogen is present.

Types of Infection

• Local Infection: Confined to a small area of the body.

• Systemic Infection: Spread throughout the body via blood or lymph.

• Focal Infection: Local infection that spreads to other parts.

• Bacteremia: Presence of bacteria in the blood.

• Septicemia: Active multiplication of bacteria in the blood; can lead to sepsis.

• Toxemia: Presence of toxins in the blood.

• Primary Infection: Initial infection by a pathogen.

• Secondary Infection: Infection by a different pathogen following a primary infection.

Stages of Disease

• Exposure: Contact with pathogen.

• Incubation: Time between exposure and onset of symptoms.

• Prodromal: Early, mild symptoms.

• Illness: Most severe symptoms.

• Decline: Symptoms begin to subside.

• Convalescence: Recovery and return to health.

Nosocomial (Hospital-Acquired) Infections

• Urinary tract infections (most common)

• Surgical site infections

• Respiratory tract infections

• Bacteremia

Isolation vs. Quarantine

• Isolation: Separates sick individuals from healthy ones; used when a person is known to be infected.

• Quarantine: Restricts movement of healthy individuals who may have been exposed; used when infection status is uncertain.

Chain/Cycle of Infection

1. Maintain Reservoir

• Human Reservoirs: Sick individuals or carriers (incubatory: infectious before symptoms; chronic: infectious for long periods). Example: Typhoid Mary.

• Animal Reservoirs: Zoonoses are diseases transmitted from animals to humans.

• Non-living Reservoirs: Food, water, soil.

2. Leave Reservoir and Enter Host (Transmission)

• Portals of Entry: Pathways by which pathogens enter the body.

• Direct Contact: Person-to-person, including fecal/oral route.

• Vehicles: Transmission via food, water, or air.

• Vectors: Arthropods (e.g., mosquitoes, ticks). Mechanical transmission: Pathogen carried on body parts. Biological transmission: Pathogen reproduces in vector.

• Respiratory Droplets: Coughing, sneezing.

• Fomites: Inanimate objects that transmit pathogens (e.g., doorknobs, towels).

• Parenteral Route: Entry through breaks in skin or mucous membranes.

3. Adhere

• Pathogens use adhesins to attach to host cells.

4. Invade

• Pathogens penetrate host tissues.

5. Evade Host Defenses

• Capsules: Prevent phagocytosis.

• Phagocytosis Resistance: Pathogens avoid destruction by immune cells.

6. Multiply

• Iron Acquisition: Pathogens require iron for growth. Siderophores are molecules that bind iron tightly, competing with host proteins. This is known as the "iron tug-of-war."

• Toxins: Exotoxins are secreted proteins (neurotoxins, cytotoxins, enterotoxins). Endotoxins are lipid A component of LPS in Gram-negative bacteria; can cause septic shock.

7. Exit (Portals of Exit)

• Pathogens leave the host via respiratory tract, gastrointestinal tract, genitourinary tract, skin, or blood.

Quantifying Virulence

• Infectious Dose 50 (ID50): Dose required to infect 50% of a population.

• Lethal Dose 50 (LD50): Dose required to kill 50% of a population.

Antimicrobial Therapy

Magic Bullet and Selective Toxicity

• Magic Bullet: Concept developed by Paul Ehrlich; a drug that targets pathogens without harming the host.

• Selective Toxicity: Ability of a drug to target pathogens with minimal harm to host cells.

Therapeutic Index

• Definition: Ratio of maximum tolerated dose to minimum effective dose against pathogen.

• Formula:

$\text{Therapeutic Index} = \frac{\text{Maximum Dose Tolerated}}{\text{Minimum Dose to Control Pathogen}}$

• A higher therapeutic index is more favorable.

Spectrum of Action

• Broad Spectrum: Effective against a wide range of pathogens.

• Narrow Spectrum: Effective against specific types of pathogens.

Antibiotics

• Produced by microbes or plants.

• Penicillin: Contains a beta-lactam ring with an R group; prevents peptide crosslinking in peptidoglycan, inhibiting cell wall synthesis.

• Penicillinase: Enzyme that breaks the beta-lactam ring, conferring resistance (common in Staphylococcus aureus).

• Routes of Administration: Intravenous (IV), intramuscular (IM), oral.

Cellular Targets of Antibiotics

• Cell wall synthesis

• Protein synthesis

• DNA synthesis

• Critical cell metabolic functions

• Cell membrane function

Antibiotic Resistance

• Resistance Genes: Encode antibiotic-degrading enzymes, efflux pumps, or altering enzymes.

• Mutations: Can prevent antibiotic entry or alter target (e.g., ribosome mutation prevents antibiotic binding).

• Multiple Resistance: Often carried on conjugative or R plasmids, allowing transfer between species.

Antiviral Drugs

• Not antibiotics; considered chemotherapeutic drugs.

• Interfere with viral replication stages: attachment, penetration, biosynthesis, release.

• Used to treat HIV, Herpes, Influenza, Hepatitis.

Review Questions and Key Concepts

• Quarantine vs. Isolation: Quarantine is for exposed but healthy individuals; isolation is for those who are sick.

• Incidence vs. Prevalence: Incidence is new cases; prevalence is total cases.

• Sporadic vs. Endemic: Sporadic is occasional; endemic is constant presence.

• Epidemic vs. Pandemic: Epidemic is local outbreak; pandemic is global.

• Bacteremia, Septicemia, Toxemia, Viremia: Presence of bacteria, actively multiplying bacteria, toxins, or viruses in the blood, respectively.

• Primary vs. Secondary Infection: Example: Influenza (primary) followed by bacterial pneumonia (secondary).

• Prodromal vs. Convalescent: Prodromal is early symptoms; convalescent is recovery phase.

• LD50: The dose that kills 50% of test subjects.

• Incubatory vs. Chronic Carrier: Incubatory is infectious before symptoms; chronic is infectious for long periods.

• Vector vs. Vehicle: Vector is a living organism (e.g., mosquito); vehicle is a non-living medium (e.g., water).

• Fomites: Inanimate objects that transmit pathogens.

• Mechanical vs. Biological Transmission: Mechanical: passive transport (e.g., fly on food); biological: pathogen develops in vector (e.g., malaria in mosquito).

• Exotoxin vs. Endotoxin: Exotoxins are secreted proteins; endotoxins are LPS components.

• Magic Bullet: Concept by Paul Ehrlich.

• Antibiotic vs. Antiviral: Antibiotics target bacteria; antivirals target viruses.

• Selective Toxicity: Drug targets pathogen, not host.

• Therapeutic Index: Ratio; higher is better.

• Spectrum of Action: Broad vs. narrow.

• Drug Toxicity: Antivirals are often more toxic due to similarities between viral and host cell processes.

• Kirby-Bauer Test: Disk diffusion test for antibiotic susceptibility; zone of inhibition indicates effectiveness.

• Drug Routes: IV (rapid, direct), IM (moderate), oral (convenient, slower).

• Penicillin: Beta-lactam ring inhibits cell wall synthesis; penicillinase confers resistance.

• Antimicrobial Targets: Cell wall, protein synthesis, DNA, metabolism, membrane.

• Antibiotic Resistance: Arises via gene transfer or mutation; mechanisms include enzyme degradation, efflux, altered targets.

• Antiviral Mechanisms: Block viral attachment, penetration, biosynthesis, release.

Table: Comparison of Disease Terms

Term	Definition	Example
Incidence	New cases in a time period	100 new flu cases in January
Prevalence	Total cases at a given time	500 flu cases in January
Sporadic	Occasional cases	Typhoid fever
Endemic	Constant presence	Malaria in Africa
Epidemic	Sudden increase	Measles outbreak
Pandemic	Global spread	COVID-19

Table: Antibiotic Resistance Mechanisms

Mechanism	Description	Example
Enzyme Degradation	Antibiotic destroyed by enzyme	Penicillinase breaks penicillin
Efflux Pumps	Antibiotic pumped out of cell	Tetracycline resistance
Altered Target	Mutation changes binding site	Ribosome mutation
Reduced Entry	Mutation prevents antibiotic entry	Porin mutation in Gram-negatives
Enzyme Modification	Antibiotic modified by enzyme	Aminoglycoside-modifying enzymes

Additional info: Academic context was added to clarify definitions, mechanisms, and examples for completeness.