Infection, Infectious Diseases, and Epidemiology: Core Concepts and Mechanisms

Symbiotic Relationships Between Microbes and Their Hosts

Types of Symbiosis

Symbiosis refers to the close association between two different biological species. In microbiology, humans maintain various symbiotic relationships with microorganisms, which can be classified as follows:

• Mutualism: Both organisms benefit. Example: Bacteria in the human colon synthesize vitamins for the host and receive nutrients in return.

• Commensalism: One organism benefits, the other is neither helped nor harmed. Example: Mites living in human hair follicles.

• Amensalism: One organism is harmed, the other is unaffected. Example: A fungus secreting antibiotics that inhibit nearby bacteria.

• Parasitism: One organism benefits at the expense of the other. Example: Mycobacterium tuberculosis in human lungs.

Type	Organism 1	Organism 2	Example
Mutualism	Benefits	Benefits	Bacteria in human colon
Commensalism	Benefits	Neither benefits nor is harmed	Mites in hair follicles
Amensalism	Is harmed	Neither benefits nor is harmed	Fungus secreting antibiotic
Parasitism	Benefits	Is harmed	Tuberculosis bacteria in lungs

Human Microbiome

The microbiome consists of all microorganisms that colonize the body's surfaces without causing disease under normal conditions. These are also called normal microbiota or indigenous microbiota and are divided into:

• Resident microbiota: Remain throughout life, mostly commensal.

• Transient microbiota: Present for short periods; eliminated by competition, body defenses, or changes in the body.

Distribution of Resident Microbiota

Resident microbiota are found in various body regions, each with characteristic genera and ecological notes:

Region	Genera	Notes
Upper Respiratory Tract	Fusobacterium, Haemophilus, Lactobacillus, etc.	Nose is cooler; microbiota of trachea/bronchi sparse; lungs nearly sterile.

Region	Genera	Notes
Upper Digestive Tract	Actinomyces, Bacteroides, Corynebacterium, etc.	Colonize teeth, gums, lining of cheeks, pharynx, etc.
Lower Digestive Tract	Bacteroides, Bifidobacterium, Clostridium, etc.	Mostly strict anaerobes; some facultative anaerobes.

Region	Genera	Notes
Female Urinary/Reproductive	Bacteroides, Clostridium, Lactobacillus, etc.	Microbiota change with vaginal pH during menstrual cycle.
Male Urinary/Reproductive	Corynebacterium, Lactobacillus, Mycobacterium, etc.	Urine flow prevents colonization of bladder/urethra.

Region	Genera	Notes
Skin	Corynebacterium, Micrococcus, Propionibacterium, etc.	Microbiota live on outer dead layers, hair follicles, and pores.
Eyes	Staphylococcus, Corynebacterium	Tears wash most microbiota from eyes.

Acquisition and Opportunism

• Humans are sterile in the womb; microbiome develops during birth and early life.

• Opportunistic pathogens are normal microbiota that cause disease under certain conditions (e.g., immune suppression, introduction to unusual sites, changes in microbiota, or stress).

Reservoirs of Infectious Diseases

Types of Reservoirs

Reservoirs are sites where pathogens are maintained as sources of infection:

• Animal reservoirs: Zoonoses are diseases naturally spread from animals to humans (e.g., rabies, malaria).

• Human carriers: Asymptomatic individuals can transmit pathogens.

• Nonliving reservoirs: Soil, water, and food contaminated with pathogens.

The Invasion and Establishment of Microbes in Hosts: Infection

Contamination vs. Infection

• Contamination: Presence of microbes on/in the body.

• Infection: Microbes evade defenses, multiply, and establish in the body.

Portals of Entry

Pathogens enter the body through specific portals:

• Skin: Entry via cuts, openings, or by burrowing/digesting outer layers.

• Mucous membranes: Line body cavities open to the environment; respiratory tract is most common entry site.

• Placenta: Usually a barrier, but some pathogens can cross and harm the fetus.

• Parenteral route: Circumvents usual portals; pathogens deposited directly into tissues (e.g., via punctures).

The Role of Adhesion in Infection

Adhesion is the process by which microbes attach to host cells, a critical step for colonization and infection. Microbes use specialized structures or attachment molecules (adhesins/ligands) to bind host cell receptors. Some bacteria form biofilms for collective attachment.

The Nature of Infectious Disease

Manifestations of Disease

• Symptoms: Subjective effects felt only by the patient (e.g., pain, nausea).

• Signs: Objective effects observed/measured by others (e.g., fever, rash).

• Syndrome: Group of symptoms and signs that characterize a disease.

• Asymptomatic (subclinical): Infections with no symptoms but possible signs.

Causation of Disease: Etiology

• Etiology: Study of the cause of disease.

• Diseases can be hereditary, congenital, degenerative, nutritional, endocrine, mental, immunological, neoplastic, infectious, iatrogenic, idiopathic, or healthcare-associated.

• Koch's postulates are used to prove a specific pathogen causes a disease.

Virulence Factors of Infectious Agents

Virulence is the degree of pathogenicity. Factors include:

• Adhesion factors

• Biofilms

• Extracellular enzymes

• Toxins (exotoxins and endotoxins)

• Antiphagocytic factors

Stages of Infectious Disease

Infectious diseases typically progress through five stages:

1. Incubation period

2. Prodromal period

3. Illness

4. Decline

5. Convalescence

The Movement of Pathogens Out of Hosts: Portals of Exit

Pathogens exit the host through portals often shared with entry points, such as the respiratory tract, gastrointestinal tract, urogenital tract, and skin.

Modes of Infectious Disease Transmission

Transmission Groups

• Contact transmission: Direct, indirect (via fomites), or droplet.

• Vehicle transmission: Airborne, waterborne, foodborne, or via bodily fluids.

• Vector transmission: Biological (e.g., mosquitoes) or mechanical (e.g., flies).

Classification of Infectious Diseases

Diseases can be classified by taxonomic category, affected body system, longevity/severity, transmission mode, or population effects. Key terms include:

• Acute: Rapid onset, short duration

• Chronic: Slow development, long duration

• Latent: Disease appears long after infection

• Communicable: Transmitted from host to host

• Noncommunicable: Not transmitted between hosts

Epidemiology of Infectious Diseases

Frequency of Disease

• Incidence: Number of new cases in a given area/time.

• Prevalence: Total number of cases in a given area/time.

Epidemiological Studies

• Descriptive epidemiology: Tabulation of data, identification of index case.

• Analytical epidemiology: Determines cause, transmission, and prevention; often retrospective.

• Experimental epidemiology: Tests hypotheses, applies Koch's postulates.

Healthcare-Associated (Nosocomial) Infections

• Exogenous: Acquired from healthcare environment.

• Endogenous: Arise from normal microbiota within patient.

• Iatrogenic: Result from medical procedures.

• Superinfections: Result from antimicrobial use disrupting normal microbiota.

• Prevention: Handwashing is most effective.

Public Health and Disease Control

• Public health agencies (e.g., CDC, WHO) monitor, educate, and enforce measures to limit disease transmission.

• Efforts include sanitation, immunization, vector control, and public education.