Infection, Infectious Diseases, and Epidemiology: Study Notes

Infection, Infectious Diseases, and Epidemiology

Symbiotic Relationships Between Microbes and Their Hosts

Microbes interact with their hosts in various ways, forming symbiotic relationships that can impact health and disease.

• Mutualism: Both organisms benefit. Example: Escherichia coli in the human gut synthesizes vitamins for the host and receives nutrients.

• Commensalism: One organism benefits, the other is unaffected. Example: Skin microbiota feeding on dead cells.

• Amensalism: One organism is harmed, the other is unaffected. Example: Antibiotic-producing microbes inhibiting nearby bacteria.

• Parasitism: The parasite benefits, the host is harmed. Disease-causing parasites are called pathogens.

Microbiome of Humans: The collection of microbes colonizing the human body without causing disease. Also known as normal microbiota, normal flora, or indigenous microbiota.

• Resident Microbiota: Present from birth to death, found on skin and mucous membranes. Mostly commensal.

• Transient Microbiota: Temporary, found in the same locations as resident microbiota.

• Acquisition: The womb is axenic; microbiota are acquired at birth and through early life exposures.

Opportunistic Pathogens: Normal microbiota can cause disease if introduced to abnormal sites, if the immune system is suppressed, or if microbial antagonism is disrupted (e.g., by antibiotics or stress).

Reservoirs of Infectious Diseases

Reservoirs are sites where pathogens are maintained and from which infection can occur.

• Animal Reservoirs: Diseases naturally spread from animals to humans (zoonoses). Transmission can occur via direct contact, consumption, or arthropod vectors.

• Human Carriers: Infected individuals or asymptomatic carriers can transmit disease. Some carriers never develop symptoms.

• Nonliving Reservoirs: Soil, water, food, and objects can harbor pathogens.

The Invasion and Establishment of Microbes in Hosts: Infection

Infection occurs when pathogens successfully invade the body, but not all infections result in disease.

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

• Infection: Successful invasion by a pathogen.

Portals of Entry

• Skin: Entry via natural openings, abrasions, or burrowing.

• Mucous Membranes: Respiratory, gastrointestinal, urinary, reproductive tracts, and conjunctiva. Respiratory tract is most common.

• Placenta: Usually a barrier, but can be breached in ~2% of pregnancies.

• Parenteral Route: Not a true portal; pathogens are deposited directly into tissues via punctures, wounds, or surgery.

The Role of Adhesion in Infection

• Adhesion: Pathogens attach to host cells using structures (e.g., suckers) or chemical receptors (adhesion factors/ligands).

• Specificity: Ligand-receptor interactions determine host specificity.

• Biofilms: Some pathogens form sticky biofilms for attachment.

The Nature of Infectious Disease

Disease (morbidity) is any change from health that interferes with normal body function. Infection and disease are distinct; infection may not always lead to disease.

Manifestations of Disease

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

• Signs: Objective, observable, measurable (e.g., fever).

• Syndrome: Group of symptoms and signs characterizing a disease (e.g., AIDS).

• Asymptomatic/Subclinical: No symptoms, but signs may be detectable.

Causation of Disease: Etiology

• Etiology: Study of the cause of disease.

• Koch’s Postulates: Four criteria to establish a microbe as the cause of a disease:

Step	Description
1	Suspected pathogen present in every case of the disease
2	Pathogen isolated and grown in pure culture
3	Pathogen causes disease when introduced into healthy host
4	Same pathogen reisolated from diseased host

• Exceptions: Some pathogens cannot be cultured, diseases may have multiple causes, or ethical constraints may prevent testing.

Virulence Factors of Infectious Agents

• Pathogenicity: Ability to cause disease.

• Virulence: Degree of pathogenicity.

• Virulence Factors: Traits enabling pathogens to cause disease:

  • Extracellular Enzymes: Dissolve structural chemicals (e.g., hyaluronidase, coagulase).

  • Toxins: Harm tissues or trigger immune responses. Exotoxins (secreted, e.g., cytotoxins, enterotoxins, neurotoxins) and Endotoxin (lipid A from Gram-negative bacteria).

  • Antiphagocytic Factors: Capsules, antiphagocytic chemicals, and leukocidins help evade immune cells.

The Stages of Infectious Diseases

After infection, disease progresses through several stages:

Stage	Description
Incubation	Time between infection and first symptoms
Prodromal	Short period of mild, generalized symptoms
Illness	Most severe stage; signs and symptoms are evident
Decline	Body returns to normal as pathogen is vanquished
Convalescence	Recovery and tissue repair

Patients may be infectious during any stage.

Movement of Pathogens Out of Hosts: Portals of Exit

Pathogens exit the host via portals such as bodily secretions, blood, wastes, and some entry portals.

Modes of Infectious Disease Transmission

Transmission is categorized into contact, vehicle, and vector modes.

• Contact Transmission:

  • Direct: Person-to-person contact.

  • Indirect: Via fomites (inanimate objects).

  • Droplet: Via droplets (<1 meter) from sneezing, coughing.

• Vehicle Transmission:

  • Airborne: Aerosols (>1 meter).

  • Waterborne: Pathogens in water (e.g., cholera).

  • Foodborne: Pathogens in food (e.g., undercooked meat).

  • Bodily Fluid: Blood, urine, etc., handled outside the body.

• Vector Transmission:

  • Biological Vectors: Arthropods serve as hosts and transmit pathogens (e.g., mosquitoes).

  • Mechanical Vectors: Arthropods passively carry pathogens (e.g., flies).

Classification of Infectious Diseases

Diseases can be classified by affected body system, taxonomic group, time course, and severity.

Type	Description	Example
Acute	Rapid onset, short duration	Common cold
Chronic	Slow onset, continual/recurrent	Tuberculosis
Subacute	Intermediate duration/severity	Additional info: Subacute bacterial endocarditis
Latent	Pathogen inactive, symptoms appear later	Herpes

• Communicable: Transmitted from infected host (e.g., measles).

• Contagious: Easily transmitted between hosts.

• Noncommunicable: Arise from outside hosts or normal microbiota (e.g., acne).

Epidemiology of Infectious Diseases

Epidemiology studies the timing, location, and transmission of diseases in populations.

Frequency of Disease

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

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

• Endemic: Disease occurs continually at stable incidence.

• Sporadic: Few scattered cases.

• Epidemic: Occurs at greater frequency than normal.

• Pandemic: Epidemic on multiple continents.

Epidemiological Studies

• Descriptive: Recording data, identifying index case.

• Analytical: Determining cause, transmission, prevention; often retrospective.

• Experimental: Testing hypotheses, applying Koch’s postulates.

Hospital Epidemiology: Healthcare-Associated (Nosocomial) Infections

Healthcare-associated infections (HAIs) are acquired in medical settings and increase morbidity and mortality.

Type	Description
Exogenous	Acquired from healthcare environment (e.g., air-conditioning, bed rails)
Endogenous	From normal microbiota becoming opportunistic
Iatrogenic	Induced by medical procedures (e.g., surgery)
Superinfection	Antibiotics disrupt normal microbiota, allowing other microbes to thrive (e.g., Clostridium difficile colitis)

• Contributing Factors: Pathogen presence, weakened immunity, transmission among staff/patients.

• Control Measures: Disinfection, antisepsis, handwashing, housekeeping, food handling.

Epidemiology and Public Health

Public health organizations collect and share epidemiological data to monitor and control disease.

• Data Sharing: Local, national, and global organizations (e.g., CDC, WHO) analyze and coordinate efforts.

• Interrupting Disease Transmission: Enforce food/water safety, reduce vectors, immunize, treat exposed individuals, establish isolation/quarantine.

• Education: Public health officials educate communities to prevent disease.

Potable water and proper food handling are essential for preventing disease.