Epidemiology and Host-Microbe Relationships: Study Guide

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Epidemiology of Infectious Diseases

Frequency of Disease

Epidemiology tracks the occurrence of diseases in populations using two primary measures: incidence and prevalence. Incidence refers to the number of new cases of a disease in a specific period, while prevalence is the total number of cases (new and existing) at a given time.

Incidence: Useful for understanding the spread of new infections.
Prevalence: Indicates the overall burden of disease in a population.
Rates and Proportions: Both measures can be expressed as rates or proportions relative to the population size.
Example: Tracking HIV/AIDS in the U.S. shows a steady increase in prevalence, while incidence remains relatively stable.

Graph comparing HIV incidence and prevalence in the U.S.

Epidemiological Reporting

Epidemiologists report disease data in various formats, such as maps, monthly case counts, and age-group distributions. This helps identify patterns and risk factors.

Geographic Distribution: Mapping cases by state or region.
Temporal Distribution: Cases reported by month or season.
Demographic Distribution: Cases per million by age group.

Epidemiological reporting: map, monthly cases, age group distribution

Tracking Disease Outbreaks

Monitoring emergency department visits and mortality rates helps identify outbreaks and epidemics. For example, influenza-like illness and deaths due to influenza and pneumonia are tracked against national baselines and epidemic thresholds.

Outbreak Detection: Sudden increases above baseline indicate potential epidemics.
Seasonal Patterns: Many infectious diseases show seasonal variation.

Graph of ER visits for influenza-like illness Graph of deaths due to influenza and pneumonia

Occurrence and Geographic Distribution

Diseases are classified based on their frequency and geographic spread:

Endemic: Occurs at a stable frequency in a given area.
Sporadic: Occurs in scattered cases.
Epidemic: Occurs at a higher than normal frequency, often as a local outbreak.
Pandemic: Epidemic occurring on multiple continents simultaneously.

Maps showing endemic, sporadic, epidemic, and pandemic disease distribution

Emerging Diseases

Emerging diseases are those whose incidence has increased in recent years or threatens to increase. Ebola virus is an example, with several outbreaks culminating in a major epidemic in 2014–15.

Graph of Ebola virus cases worldwide

Nationally Notifiable Infectious Diseases

Certain infectious diseases must be reported to public health authorities at local, state, and national levels. The CDC maintains a list of nationally notifiable diseases.

Disease	Disease
Acquired immunodeficiency syndrome (AIDS)	Pertussis
Anthrax	Plague
Arboviral diseases	Poliomyelitis, paralytic
Botulism	Q fever
Brucellosis	Rabies, animal and human
Chancroid	Rocky Mountain spotted fever
Chlamydia trachomatis, genital infections	Rubella
Cholera	Rubella, congenital syndrome
Coccidioidomycosis	Salmonellosis
Cryptosporidiosis	Severe acute respiratory syndrome (SARS)
Cyclosporiasis	Shiga-toxin-producing Escherichia coli
Diphtheria	Shigellosis
Ehrlichiosis	Smallpox
Giardiasis

Table of nationally notifiable infectious diseases

Disease	Disease
Haemophilus influenzae, invasive disease	Streptococcal disease, invasive, Group A
Hansen disease (leprosy)	Streptococcal toxic shock syndrome
Hantavirus pulmonary syndrome	Streptococcus pneumoniae, invasive disease
Hemolytic uremic syndrome, postdiarrheal	Syphilis
Hepatitis A, B, C	Tetanus
HIV infection	Toxic-shock syndrome
Influenza-associated pediatric deaths	Tuberculosis
Legionellosis	Typhoid fever
Lyme disease	Varicella
Malaria	West Nile virus disease
Meningococcal disease
Mumps

Table of nationally notifiable infectious diseases (continued)

Descriptive Epidemiology

Descriptive epidemiology involves identifying the index case and mapping disease incidence to determine the source and spread of outbreaks. John Snow's investigation of cholera in London is a classic example.

Index Case: The first documented patient in an outbreak.
Mapping: Helps identify sources and transmission routes.

John Snow and map of cholera cases

Analytical and Experimental Epidemiology

Analytical epidemiology investigates disease causes and risk factors, often retrospectively. Experimental epidemiology tests hypotheses, sometimes using Koch’s postulates to confirm causative agents.

Analytical Epidemiology: Retrospective studies to identify risk factors.
Experimental Epidemiology: Controlled experiments to test hypotheses about disease causation.

Nosocomial Infections (Hospital-Acquired)

Nosocomial infections are acquired in healthcare settings. They can be exogenous (from external sources), endogenous (from the patient’s own flora), or iatrogenic (resulting from medical procedures).

Exogenous: Pathogens from the hospital environment.
Endogenous: Patient’s own microbes cause infection.
Iatrogenic: Caused by medical interventions.

Factors Influencing Nosocomial Infections

Depressed host defenses (disease, malnutrition, age)
Breached anatomic barriers (IVs, catheters, vents)
Exposure to virulent, often antibiotic-resistant pathogens

Venn diagram of nosocomial infection factors

Public Health and Epidemiology

Public health agencies at local, national, and global levels coordinate disease surveillance and response. The Department of Health and Human Services (U.S.) and the World Health Organization (WHO) are key agencies.

Host-Microbe Relationships

Symbiosis

Host-microbe relationships can be mutualistic, commensal, or parasitic. Pathogenicity refers to a microbe’s ability to cause disease, while virulence is the degree of pathogenicity.

Pathogenicity: Ability to cause disease.
Virulence: Severity of disease caused.

Normal Microflora

Normal microflora are organisms that live on or in the body without causing disease. They are classified as resident (always present) or transient (temporarily present).

Resident Microflora: Stable populations in specific body sites.
Transient Microflora: Temporary, often removed by host defenses.

Diagram of resident microflora locations

Body Site	Genera	Notes
Upper Respiratory Tract	Fusobacterium, Haemophilus, Lactobacillus, Neisseria, Staphylococcus, Streptococcus, Candida	Similar to oral cavity; lungs are usually microbe-free.
Upper Digestive Tract	Actinomyces, Bacteroides, Haemophilus, Lactobacillus, etc.	Microbes colonize surfaces of teeth, gums, tongue, and cheeks.
Lower Digestive Tract	Bacteroides, Clostridium, Enterococcus, etc.	Lower tract has many strict anaerobes.
Female Urinary/Reproductive	Bacteroides, Clostridium, Lactobacillus, Staphylococcus, Candida	Microbial populations change with activity and hormones.
Male Urinary/Reproductive	Bacteroides, Pseudomonas, Staphylococcus, etc.	Urine is sterile until it exits the urethra.
Eye and Skin	Staphylococcus, Micrococcus, Propionibacterium, etc.	Microbes live on the outer skin and conjunctiva.

Table of resident microflora in upper respiratory tract

Microbe-Free Areas

Certain body sites are normally microbe-free (axenic) due to physical barriers, unsuitable conditions, or host defenses.

Internal Tissues and Organs	Body Fluids
Middle and inner ear, sinuses, internal eye, bone marrow, muscles, glands, organs, circulatory system, brain and spinal cord, ovaries and testes	Blood, cerebrospinal fluid, saliva prior to secretion, urine in kidneys/bladder, semen prior to urethra

Table of body sites normally microbe-free

Reservoirs of Infectious Disease

Animal Reservoirs

Animal reservoirs are sources of zoonotic diseases, which are transmitted from animals to humans. Examples include rabies, malaria, and ringworm.

Disease	Causative Agent	Animal Reservoir	Mode of Transmission
Tapeworm infection	Dipylidium caninum	Dogs	Ingestion of contaminated food
Malaria	Plasmodium spp.	Monkeys	Bite of Anopheles mosquito
Ringworm	Trichophyton spp.	Domestic animals	Direct contact
Rabies	Rabies virus	Domestic and wild animals	Bite of infected animal

Table of common zoonoses

Human and Nonliving Reservoirs

Human Reservoirs: Carriers may transmit disease without symptoms.
Nonliving Reservoirs: Soil, water, and food can harbor pathogens.

Contamination, Infection, and Disease

Process of Infection

Infection begins with contamination, followed by invasion and multiplication of pathogens, leading to disease if host defenses are overcome.

Portals of Entry

Pathogens enter the body through specific portals, including skin, mucous membranes, placenta, and parenteral routes (e.g., injections, wounds).

Diagram of portals of entry for pathogens Diagram of male and female portals of entry

Pathogen	Condition in Adult	Effect on Embryo or Fetus
Toxoplasma gondii	Toxoplasmosis	Abortion, epilepsy, mental retardation, etc.
Treponema pallidum	Syphilis	Abortion, mental retardation, etc.
Listeria monocytogenes	Listeriosis	Abortion, neonatal death, etc.
Cytomegalovirus	Usually asymptomatic	Deafness, microcephaly, mental retardation
HIV	AIDS	Immunosuppression, death
Rubella virus	German measles	Severe birth defects, death

Table of pathogens that cross the placenta

Nature of Infectious Disease

Signs, Symptoms, and Syndromes

Diseases are characterized by signs (observable/measurable), symptoms (felt by the patient), and syndromes (groups of signs and symptoms).

Symptoms (patient)	Signs (observer)
Pain, nausea, headache, chills, sore throat, fatigue, malaise, discomfort, itching, abdominal cramps	Swelling, rash, vomiting, diarrhea, fever, pus, anemia, leukocytosis/leukopenia, bubo, tachycardia/bradycardia

Table of typical manifestations of disease

Etiology: Cause of Disease

Infectious vs Noninfectious: Infectious diseases are caused by pathogens; noninfectious by other factors.
Communicable vs Noncommunicable: Communicable diseases spread from person to person; noncommunicable do not.

Koch's Postulates

Koch’s postulates are criteria used to prove that a specific pathogen causes a particular disease:

The suspected agent must be present in every case of the disease.
The agent must be isolated and grown in pure culture.
The cultured agent must cause the disease when inoculated into a healthy host.
The same agent must be reisolated from the experimental host.

Diagram of Koch's postulates

How Microbes Cause Disease

Virulence Factors

Virulence factors are traits that enable pathogens to cause disease, including adhesion, invasion, and toxin production.

Adhesion

Pathogens use adhesins (ligands) to attach to host cell receptors, a critical step in infection.

Diagram of microbial adhesion to host cell

Invasion and Exoenzymes

Pathogens invade tissues using exoenzymes such as hyaluronidase and collagenase, which break down host barriers.

Diagram of collagenase action in blood vessel invasion Diagram of H. pylori invasion of stomach lining

Toxins

Bacterial toxins are classified as endotoxins (lipopolysaccharide, LPS, from Gram-negative bacteria) and exotoxins (secreted proteins from Gram-positive and some Gram-negative bacteria).

Diagram of endotoxin structure

Feature	Exotoxin	Endotoxin
Source	Gram-positive and Gram-negative bacteria	Gram-negative bacteria
Relation to bacteria	Secreted	Part of cell wall, released upon cell death
Chemistry	Protein	Lipid A of LPS
Toxicity	High	Low
Heat stability	Unstable	Stable
Effect on host	Variable	Fever, inflammation, shock
Fever producing?	No	Yes
Antigenicity	Strong	Weak
Diseases	Diphtheria, tetanus, cholera	Typhoid fever, meningitis

Table comparing exotoxins and endotoxins Diagram of exotoxin and endotoxin effects

Immune Evasion

Pathogens evade host immunity using antiphagocytic factors (capsules, slime layers), proteases, coagulase, kinase, and antigenic variation.

Micrograph and diagram of bacterial capsules and protease action Diagram of coagulase and kinase action

Antigenic Variation

Some pathogens alter their surface proteins to avoid immune detection. Examples include Neisseria gonorrhoeae, Borrelia burgdorferi, and Trypanosoma species.

Diagram of antigenic drift and shift in influenza viruses

Eukaryotic Pathogen Virulence

Fungi: Adhesins, exoenzymes, mycotoxins, capsule production (e.g., Candida).
Protozoa: Adhesive disks (Giardia intestinalis), capsule production, antigenic variation (Trypanosoma brucei, Plasmodium falciparum).
Helminths: Adhesion (hooks, suckers), invasion (Schistosoma mansoni), immune evasion (size, cuticle protection).

Micrograph of tapeworm hooks and suckers Schistosoma life cycle diagram Micrograph of roundworm with tough cuticle

Stages of Infectious Disease

Progression of Disease

The course of an infectious disease is divided into five periods: incubation, prodromal, illness, decline, and convalescence. The number of pathogens and severity of symptoms vary throughout these stages.

Incubation: No signs or symptoms.
Prodromal: Mild, vague symptoms.
Illness: Most severe signs and symptoms.
Decline: Symptoms and pathogen numbers decrease.
Convalescence: Recovery, no symptoms.

Graph of infectious disease stages

Portals of Exit

Pathogens exit the host through various portals, including skin, respiratory tract, gastrointestinal tract, and genitourinary tract, to infect new hosts.

Diagram of portals of exit for pathogens

Modes of Infectious Disease Transmission

Contact Transmission

Direct: Physical contact between individuals.
Indirect: Transmission via fomites (objects).
Droplet: Transmission via respiratory droplets.

Vehicle Transmission

Pathogens transmitted via air, water, or food.

Vector Transmission

Biological Vector: Pathogen develops in vector (e.g., mosquito).
Mechanical Vector: Pathogen carried on vector’s body (e.g., fly).