Infection, Infectious Diseases, and Epidemiology

Symbiotic Relationships Between Microbes and Their Hosts

Microorganisms interact with their hosts in various symbiotic relationships, which influence health and disease.

• Symbiosis: Means "to live together"; refers to the close association between two different species.

• Types of Symbiosis:

  • Mutualism: Both organisms benefit.

  • Commensalism: One organism benefits, the other is unharmed.

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

  • Parasitism: One organism benefits (the parasite), the other is harmed (the host).

• Example: Escherichia coli in the human gut (mutualism).

Normal Microbiota in Hosts

Normal microbiota are microorganisms that colonize the body's surfaces without normally causing disease.

• Also called normal flora, indigenous microbiota, or microbiome.

• Two types:

  • Resident microbiota: Remain part of the microbiota throughout life; mostly commensal.

  • Transient microbiota: Remain in the body for a short period; found in the same regions as resident microbiota but cannot persist due to competition, elimination by defense cells, or chemical/physical changes.

Acquisition of Normal Microbiota

Humans develop microbiota after birth, as the womb is typically free of microorganisms.

• Microbiota begin to develop during the birthing process.

• Most resident microbiota are established during the first months of life.

• Additional info: Recent research questions the "sterile womb" hypothesis, suggesting possible in utero colonization.

How Normal Microbiota Become Opportunistic Pathogens

Normal microbiota can cause disease under certain conditions, becoming opportunistic pathogens.

• Opportunistic pathogens arise when:

  1. Normal microbiota are introduced into unusual sites in the body.

  2. Immune suppression occurs.

  3. Relative abundance of normal microbiota changes.

  4. Stressful conditions affect the host.

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 animal hosts to humans (e.g., rabies, malaria).

• Human reservoirs: Human carriers may be asymptomatic but still infectious (e.g., Typhoid Mary).

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

Table: Some Common Zoonoses

The Invasion and Establishment of Microbes in Hosts

Exposure to Microbes: Contamination and Infection

Pathogens must enter the host to cause infection.

• Contamination: Mere presence of microbes in or on the body.

• Infection: Microorganism evades external defenses, multiplies, and becomes established.

Portals of Entry

Pathogens enter the body through specific sites called portals of entry.

• Skin: Acts as a barrier; pathogens may enter through cuts, abrasions, or by burrowing/digesting outer layers.

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

• Placenta: Typically forms a barrier, but some pathogens can cross and infect the fetus, causing birth defects or abortion.

• Parenteral route: Not a true portal; pathogens are deposited directly into tissues beneath the skin or mucous membranes (e.g., via punctures, bites).

Table: Some Pathogens That Cross the Placenta

The Role of Adhesion in Infection

Adhesion is the process by which microorganisms attach to host cells, a critical step for colonization and infection.

• Uses adhesion factors such as specialized structures and attachment molecules.

• Attachment proteins (found on viruses and bacteria) bind to host cell receptors.

• Blocking these interactions can prevent infection.

• Some bacteria form biofilms for collective attachment.

The Nature and Manifestations of Infectious Disease

Definitions

• Infection: Invasion of the host by a pathogen.

• Disease: Results if the pathogen alters normal body functions; also called morbidity.

Manifestations of Disease: Symptoms, Signs, and Syndromes

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

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

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

• Asymptomatic (subclinical) infections: Lack symptoms but may show signs.

Table: Terminology of Disease

Etiology

Etiology is the study of the cause of disease. Diseases can have hereditary, congenital, nutritional, infectious, or other origins.

Table: Categories of Disease

Koch's Postulates

Koch's postulates are criteria used to establish a causative relationship between a microbe and a disease.

• Microorganism must be found in all cases of the disease.

• It must be isolated and grown in pure culture.

• It must cause the disease when introduced into a healthy host.

• It must be re-isolated from the experimentally infected host.

Exceptions: Some pathogens cannot be cultured, diseases may have multiple causes, and ethical issues may prevent testing in humans.

Virulence Factors of Infectious Agents

Definitions

• Pathogenicity: Ability of a microorganism to cause disease.

• Virulence: Degree of pathogenicity; determined by specific factors.

Virulence Factors

• Adhesion factors: Enable attachment to host cells.

• Biofilms: Communities of microorganisms attached to surfaces.

• Extracellular enzymes: Secreted to dissolve host chemicals and aid invasion.

• Toxins: Harm host tissues or trigger damaging immune responses.

  • Exotoxins: Secreted proteins causing specific effects (e.g., botulinum toxin).

  • Endotoxins: Lipopolysaccharides from Gram-negative bacteria released upon cell death.

• Antiphagocytic factors: Prevent destruction by host phagocytes (e.g., bacterial capsule, antiphagocytic chemicals).

The Stages of Infectious Disease

Infectious diseases typically progress through five stages:

• Incubation period: No signs or symptoms.

• Prodromal period: Vague, general symptoms.

• Illness: Most severe signs and symptoms.

• Decline: Declining signs and symptoms.

• Convalescence: No signs or symptoms; recovery.

Table: Incubation Periods of Selected Infectious Diseases

The Movement of Pathogens Out of Hosts: Portals of Exit

Pathogens leave the host through portals of exit, often the same as portals of entry, and are commonly found in body secretions or excretions.

• Examples: Saliva, mucus, blood, urine, feces, vaginal secretions.

Transmission of Infectious Diseases

Modes of Transmission

Transmission occurs from a reservoir or portal of exit to another host's portal of entry.

• Contact transmission:

  • Direct contact: Physical contact between hosts.

  • Indirect contact: Pathogens spread via fomites (inanimate objects).

  • Droplet transmission: Spread via droplets of mucus (exhaling, coughing, sneezing).

• Vehicle transmission: Pathogens transmitted by air, water, or food.

• Vector transmission: Arthropods transmit pathogens (e.g., mosquitoes, ticks).

Classification of Infectious Diseases

Diseases can be classified by the body system affected, longevity/severity, mode of transmission, and population effects.

Epidemiology of Infectious Diseases

Definitions

• Epidemiology: Study of when and where diseases occur and how they are transmitted.

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

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

Epidemiological Studies

• Analytical epidemiology: Determines probable cause, mode of transmission, and prevention methods; often retrospective.

• Hospital epidemiology: Focuses on healthcare-associated (nosocomial) infections; aggressive control measures such as handwashing are essential.

• Public health organizations: Share data at local, national, and global levels (e.g., CDC, WHO).