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

Symbiotic Relationships Between Microbes and Their Hosts

Types of Symbiosis

Symbiosis refers to the close association between two different biological species. In microbiology, symbiotic relationships between microbes and their hosts are fundamental to understanding health and disease.

• Mutualism: Both organisms benefit from the relationship.

• Commensalism: One organism benefits, while the other is neither helped nor harmed.

• Amensalism: One organism is harmed, and the other is unaffected.

• Parasitism: One organism (the parasite) benefits at the expense of the other (the host).

Example: Termites and their gut protozoa exhibit mutualism, where protozoa digest cellulose for the termite, and in return, receive shelter and nutrients.

Normal Microbiota in Hosts

Definition and Types

Normal microbiota (also called normal flora or indigenous microbiota) are microorganisms that colonize the body's surfaces without causing disease under normal conditions. They are essential for health, aiding in digestion, immunity, and protection against pathogens.

• Resident microbiota: Remain a part of the normal microbiota throughout life and are mostly commensal.

• Transient microbiota: Present temporarily; cannot persist due to competition, immune responses, or physical/chemical changes in the body.

Distribution of Resident Microbiota

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

Body Site	Genera	Notes
Upper Digestive Tract	Actinomyces, Bacteroides, Corynebacterium, Haemophilus, Lactobacillus, Neisseria, Staphylococcus, Streptococcus, Treponema, Entamoeba, Trichomonas	Microbes colonize surfaces of teeth, gums, lining of cheeks, and throat; many species in saliva.
Lower Digestive Tract	Bacteroides, Clostridium, Escherichia, Lactobacillus, Proteus, Shigella, Candida, Entamoeba, Trichomonas	Mostly strict anaerobes; some are facultative anaerobes.

Body Site	Genera	Notes
Upper Respiratory Tract	Fusobacterium, Haemophilus, Lactobacillus, Moraxella, Staphylococcus, Streptococcus, Candida	Microbiota in nose and mouth; lower respiratory tract is nearly sterile.

Body Site	Genera	Notes
Female Urinary and Reproductive Systems	Bacteroides, Clostridium, Lactobacillus, Staphylococcus, Streptococcus, Candida	Microbiota change with menstrual cycle; urine is sterile except at distal end of urethra.
Male Urinary and Reproductive Systems	Bacteroides, Lactobacillus, Mycobacterium, Staphylococcus, Streptococcus	Flow of urine prevents colonization except at distal end of urethra.

Body Site	Genera	Notes
Eyes and Skin	Corynebacterium, Micrococcus, Propionibacterium, Staphylococcus, Candida, Malassezia	Microbes live on outer dead layers of skin and conjunctiva; tears wash most microbes from eyes.

Acquisition of Normal Microbiota

Humans develop in a sterile environment in the womb. Microbiota begin to colonize the body during birth and are established in the first months of life.

Opportunistic Pathogens

Normal microbiota can become opportunistic pathogens under certain conditions, such as:

• Introduction into unusual body sites

• Immune suppression

• Changes in abundance of normal microbiota

• Stressful conditions

Reservoirs of Infectious Diseases of Humans

Types of Reservoirs

Reservoirs are sites where pathogens are maintained as sources of infection. They include:

• Animal reservoirs: Zoonoses are diseases naturally spread from animals to humans via direct contact, consumption, or vectors.

• Human carriers: Asymptomatic individuals can transmit pathogens.

• Nonliving reservoirs: Soil, water, and food contaminated by feces or urine.

Common Zoonoses

Disease	Causative Agent	Animal Reservoir	Mode of Transmission
Tapeworm infestation	Dipylidium caninum	Dogs	Ingestion of larvae in dog saliva
Malaria	Plasmodium spp.	Monkeys	Bite of Anopheles mosquito
Ringworm	Microsporum	Domestic animals	Direct contact
Anthrax	Bacillus anthracis	Livestock	Direct contact, inhalation
Rabies	Lyssavirus	Bats, dogs, foxes	Bite of infected animal

The Invasion and Establishment of Microbes in Hosts: Infection

Exposure to Microbes: Contamination and Infection

• Contamination: Presence of microbes on body surfaces.

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

Portals of Entry

Pathogens enter the body through specific sites:

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

• Mucous membranes: Respiratory tract (most common), gastrointestinal tract (must survive stomach acid).

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

• Parenteral route: Circumvents usual portals; pathogens deposited directly into tissues.

Pathogen	Condition in Adult	Effect on Embryo or Fetus
Toxoplasma gondii	Toxoplasmosis	Abortion, epilepsy, microcephaly, mental retardation, blindness
Treponema pallidum	Syphilis	Abortion, birth defects, syphilis
Listeria monocytogenes	Listeriosis	Granulomatosis infantiseptica, death
Cytomegalovirus	Usually asymptomatic	Deafness, microcephaly, mental retardation
Rubivirus	German measles	Severe birth defects, death

The Role of Adhesion in Infection

Adhesion is the process by which microorganisms attach to host cells, a critical step for colonization and infection. Microbes use specialized structures or attachment molecules (adhesins/ligands) to bind to host cell receptors. Disruption of these interactions can prevent infection. Some bacteria form biofilms for enhanced attachment and survival.

The Nature of Infectious Disease

Manifestations of Disease

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

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

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

• Asymptomatic (subclinical) infections: No symptoms, but signs may be present.

Terminology of Disease

Prefix/Suffix	Meaning	Example
carcino-	Cancer	Carcinogenic: giving rise to cancer
itis	Inflammation	Colitis: inflammation of the colon
emia	Pertaining to the blood	Viremia: viruses in the blood
oma	Tumor	Neoplasm: benign or malignant tumor
patho-	Disease	Pathogenic: causing disease
toxi-	Poison	Toxin: harmful compound

Etiology and Koch’s Postulates

Etiology is the study of the cause of disease. Koch's postulates are criteria used to establish a causative relationship between a microbe and a disease:

1. The suspected agent must be present in every case of the disease.

2. The agent must be isolated and grown in pure culture.

3. The cultured agent must cause the disease when inoculated into a healthy host.

4. The same agent must be reisolated from the diseased experimental host.

Exceptions exist, such as pathogens that cannot be cultured, diseases with multiple causes, or ethical limitations.

Virulence Factors of Infectious Agents

Definitions

• Pathogenicity: Ability of a microorganism to cause disease.

• Virulence: Degree of pathogenicity, determined by virulence factors.

Virulence factors include adhesion factors, biofilms, extracellular enzymes, toxins, and antiphagocytic factors.

Extracellular Enzymes

Secreted by pathogens to dissolve host structural chemicals, aiding invasion and evasion of defenses. Mutants lacking these enzymes are often avirulent.

Toxins

Toxins are chemicals that damage host tissues or trigger harmful immune responses. Toxemia is the presence of toxins in the bloodstream. There are two main types:

• Exotoxins: Secreted proteins, highly toxic, specific effects.

• Endotoxins: Lipid A component of Gram-negative bacteria outer membrane, released upon cell death, less specific effects.

Feature	Exotoxins	Endotoxins
Source	Gram-positive and Gram-negative bacteria	Gram-negative bacteria
Chemical Nature	Protein or short peptide	Lipid A of LPS
Toxicity	High	Low
Effect on Host	Variable, specific	Fever, shock, blood coagulation
Antigenicity	Strong	Weak

Antiphagocytic Factors

These factors help pathogens evade phagocytosis by host immune cells, allowing prolonged infection. Examples include bacterial capsules (which are not recognized as foreign and are slippery) and chemicals that prevent lysosome fusion or destroy phagocytes (leukocidins).

The Stages of Infectious Disease

Infectious diseases typically progress through five stages:

1. Incubation period

2. Prodromal period

3. Illness

4. Decline

5. Convalescence

Disease	Incubation Period
Staphylococcus foodborne infection	<1 day
Influenza	About 1 day
Cholera	2 to 3 days
Genital herpes	About 5 days
Tetanus	5 to 15 days
Syphilis	10 to 21 days
Hepatitis B	70 to 100 days
AIDS	1 to >8 years
Leprosy	10 to >30 years

The Movement of Pathogens Out of Hosts: Portals of Exit

Pathogens exit the host through portals of exit, often the same as portals of entry, via secretions or excretions (e.g., respiratory droplets, feces, urine).

Transmission of Infectious Diseases

Contact Transmission

• Direct contact: Physical interaction (e.g., touching, kissing).

• Indirect contact: Via fomites (inanimate objects).

• Droplet transmission: Via respiratory droplets over short distances.

Vehicle Transmission

• Spread via air, water, or food.

Vector Transmission

• Spread by living organisms (e.g., mosquitoes, ticks).

Classification of Infectious Diseases

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

Term	Definition
Acute disease	Rapid onset, short duration
Chronic disease	Slow onset, long duration
Latent disease	Inactive for a time, then active
Communicable disease	Spread from host to host
Contagious disease	Easily spread
Noncommunicable disease	Not spread from person to person
Systemic infection	Widespread in body
Focal infection	Confined, but can spread
Primary infection	Initial infection
Secondary infection	Follows a primary infection

Epidemiology of Infectious Diseases

Definitions and Measures

Epidemiology is the study of when, where, and how diseases occur and are transmitted. Key measures include:

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

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

Reporting and Data Representation

Epidemiologists use various graphical and tabular methods to report disease data, such as maps, curves, and bar charts.

Patterns of Disease Occurrence

• Endemic: Disease normally present in a population.

• Sporadic: Occasional cases.

• Epidemic: Higher than normal frequency.

• Pandemic: Epidemic on multiple continents.

Nationally Notifiable Infectious Diseases

Certain diseases must be reported to public health authorities for monitoring and control.

Epidemiological Studies

Types of Studies

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

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

Hospital Epidemiology: Healthcare-Associated (Nosocomial) Infections

Types of Healthcare-Associated Infections

• Exogenous: Acquired from healthcare environment.

• Endogenous: Arise from patient's own microbiota.

• Iatrogenic: Result from medical procedures.

• Superinfections: Result from antimicrobial use disrupting normal microbiota.

Control: Aggressive measures, especially handwashing, are essential to reduce healthcare-associated infections.

Epidemiology and Public Health

Public health agencies at local, national, and global levels (e.g., CDC, WHO) coordinate efforts to monitor, report, and control infectious diseases.