Chapter 14: Infection, Infectious Diseases, and Epidemiology

Infection, Infectious Diseases, and Epidemiology

Symbiotic Relationships Between Microbes and Their Hosts

Symbiosis refers to the close association between two different organisms living together. In microbiology, humans have various symbiotic relationships with microorganisms, which can be classified based on the effects on each partner.

• Mutualism: Both organisms benefit. Example: Bacteria in the human colon.

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

• Amensalism: One organism is harmed, the other is unaffected. Example: Fungus secreting an antibiotic, inhibiting nearby bacteria.

• Parasitism: One organism benefits at the expense of the other. Example: Tuberculosis bacteria in human lung.

	Organism 1	Organism 2	Example
Mutualism	Benefits	Benefits	Bacteria in human colon
Commensalism	Benefits	Neither benefits nor is harmed	Mites in human hair follicles
Amensalism	Is harmed	Neither benefits nor is harmed	Fungus secreting an antibiotic, inhibiting nearby bacteria
Parasitism	Benefits	Is harmed	Tuberculosis bacteria in human lung

Example of Mutualism

Termites cannot digest cellulose in wood. Protozoans in their intestines digest cellulose with the help of bacteria inside the protozoans. All three organisms benefit, illustrating a mutualistic relationship.

The Human Microbiome

The human microbiome consists of all microorganisms that colonize the body's surfaces without causing disease. These are also called normal microbiota, normal flora, or indigenous microbiota. There are two main types:

• Resident microbiota: Remain throughout life, mostly commensal, and found in specific body regions.

• Transient microbiota: Present for short periods (hours to months), often eliminated by competition, body defenses, or changes in the body.

Example: Bacteria colonizing the surface of human nasal cells.

Resident Microbiota by Body Site

• Upper Respiratory Tract: Genera include Staphylococcus, Streptococcus, Haemophilus, Neisseria, Candida (fungus). The nose is cooler and has unique microbiota compared to the lower respiratory tract.

• Upper Digestive Tract: Includes Actinomyces, Bacteroides, Corynebacterium, Lactobacillus, etc. Microbes colonize teeth, gums, and cheeks.

• Lower Digestive Tract: Includes Bacteroides, Bifidobacterium, Clostridium, etc. Most are anaerobes.

• Urinary and Reproductive Systems: Female: Bacteroides, Clostridium, Lactobacillus, Candida (yeast). Male: Bacteroides, Lactobacillus, Mycobacterium, etc. Microbiota change with activity and urine flow.

• Eyes and Skin: Staphylococcus, Micrococcus, Propionibacterium, Candida (fungus). Microbes adhere to the outer dead layers of skin and hair follicles.

Development of the Microbiome

The human microbiome begins to develop during the birthing process. Babies born vaginally develop a healthier microbiome earlier. Diet and breastfeeding also influence microbiome development and are associated with reduced risk of atopic (allergic) disorders.

Opportunistic Pathogens

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

• Introduction into unusual body sites

• Immune suppression

• Changes in the normal microbiota (e.g., antibiotic use)

• Stressful conditions

Reservoirs of Infectious Diseases

Reservoirs are sites where pathogens are maintained as sources of infection. There are three main types:

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

• Human carriers: Asymptomatic individuals can transmit pathogens (e.g., tuberculosis, syphilis, AIDS).

• Nonliving reservoirs: Soil, water, and food contaminated with pathogens (e.g., Clostridium in soil).

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	Trichophyton spp.	Domestic animals	Direct contact
Anthrax	Bacillus anthracis	Domestic livestock	Contact/inhalation
Rabies	Lyssavirus sp.	Bats, skunks, foxes, dogs	Bite of infected animal

Invasion and Establishment of Microbes in Hosts: Infection

Infection occurs when an organism evades the body's external defenses, multiplies, and becomes established. Entry occurs through specific portals:

• Skin: Barrier to most microbes; entry via cuts, hair follicles, or by burrowing/digesting outer layers.

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

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

• Parenteral route: Pathogens deposited directly into tissues beneath the skin or mucous membranes (e.g., punctures).

Pathogen	Condition in the Adult	Effect on Embryo or Fetus
Toxoplasma gondii	Toxoplasmosis	Abortion, epilepsy, encephalitis, microcephaly, mental retardation, blindness, anemia, etc.
Treponema pallidum	Syphilis	Abortion, birth defects, syphilis
Listeria monocytogenes	Listeriosis	Granulomatosis infantiseptica, infant blood poisoning
Cytomegalovirus	Usually asymptomatic	Deafness, microcephaly, mental retardation
Lentivirus (HIV)	AIDS	Immunosuppression (AIDS)
Rubivirus	German measles	Severe birth defects or death

The Nature of Infectious Disease

Infection is the invasion of the host by a pathogen. Disease (morbidity) results if the pathogen alters normal body functions. Manifestations of disease include:

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

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

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

• Asymptomatic (subclinical): Infections without symptoms but with detectable signs.

Symptoms (Sensed by the Patient)	Signs (Detected or Measured by an Observer)
Pain, nausea, headache, chills, sore throat, fatigue, malaise, itching, abdominal cramps	Swelling, rash, vomiting, diarrhea, fever, pus, anemia, leukocytosis/leukopenia, bubo, tachycardia/bradycardia

Etiology: Causation of Disease

Etiology is the study of the cause of disease. Robert Koch developed postulates to prove that a specific pathogen causes a specific 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 include pathogens that cannot be cultured, diseases caused by multiple agents, and ethical limitations.

Categories of Diseases

Description	Examples
Hereditary	Sickle-cell anemia, diabetes mellitus
Congenital	Fetal alcohol syndrome, deafness from rubella
Degenerative	Renal failure, age-related farsightedness
Nutritional	Kwashiorkor, rickets
Endocrine (hormonal)	Addison's disease
Mental	Skin rash, gastrointestinal distress
Immunological	Allergies, autoimmune diseases
Neoplastic (tumor)	Benign tumors, cancers
Infectious	Colds, influenza, herpes
Iatrogenic	Surgical error, yeast vaginitis from therapy
Idiopathic	Alzheimer's disease, multiple sclerosis
Healthcare-Associated (nosocomial)	Pseudomonas infection in burn patient

Virulence Factors of Infectious Agents

Virulence is the degree of pathogenicity. Virulence factors include:

• Adhesion factors

• Biofilms

• Extracellular enzymes

• Toxins

• Antiphagocytic factors

Relative virulence varies among pathogens.

Extracellular Enzymes

Pathogens secrete enzymes that dissolve structural chemicals in the body, aiding invasion and evasion of defenses. Mutants lacking these enzymes are often avirulent.

Toxins

Toxins are chemicals that harm tissues or trigger damaging immune responses. Toxemia is the presence of toxins in the bloodstream. Two main types:

• Exotoxins: Secreted by living bacteria; highly toxic.

• Endotoxins: Released from dead Gram-negative bacteria (lipid A of LPS); can cause fever, shock.

	Exotoxins	Endotoxins
Source	Mainly Gram-positive and Gram-negative bacteria	Gram-negative bacteria
Nature	Protein or short peptide	Lipid portion (lipid A) of LPS
Toxicity	High	Low but fatal in high doses
Heat Stability	Unstable at 60°C	Stable at 121°C
Effect on Host	Variable (neurotoxin, enterotoxin, cytotoxin)	Fever, malaise, shock, blood coagulation
Antigenicity	Strong	Weak
Toxoid Formation	Possible	Not feasible
Representative Diseases	Tetanus, diphtheria, cholera	Typhoid fever, meningococcal meningitis

Antiphagocytic Factors

These factors prevent phagocytosis by host cells, allowing pathogens to persist. Examples include bacterial capsules (not recognized as foreign, slippery) and chemicals that prevent lysosome fusion or destroy white blood cells.

Stages of Infectious Disease

Infectious diseases typically progress through five stages:

1. Incubation period: No signs or symptoms.

2. Prodromal period: Vague, general symptoms.

3. Illness: Most severe signs and symptoms.

4. Decline: Declining signs and symptoms.

5. Convalescence: No signs or symptoms.

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

Portals of Exit

Pathogens leave the host through various portals, often the same as entry points, including secretions, excretions, and body fluids.

Modes of Infectious Disease Transmission

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

• Contact transmission: Direct (body contact), indirect (fomites), droplet (mucus droplets <1 meter).

• Vehicle transmission: Airborne (>1 meter via aerosols), waterborne (fecal-oral), foodborne, bodily fluids.

• Vector transmission: Biological (arthropods as hosts), mechanical (passive transfer, e.g., houseflies).

Disease	Causative Agent	Vector
Malaria	Plasmodium spp.	Anopheles mosquito
Yellow fever	Flavivirus	Aedes mosquito
Lyme disease	Borrelia burgdorferi	Ixodes tick
Bubonic plague	Yersinia pestis	Xenopsylla flea
Epidemic typhus	Rickettsia prowazekii	Pediculus louse

Classification of Infectious Diseases

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

Epidemiology of Infectious Diseases

Epidemiology studies the frequency, distribution, and control of diseases. Key measures include:

• 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 include descriptive (tabulation of data), analytical (cause, transmission, prevention), and experimental (testing hypotheses).

Nosocomial (Healthcare-Associated) Infections

Types include:

• Exogenous: Acquired from the healthcare environment.

• Endogenous: Arise from normal microbiota within the patient.

• Iatrogenic: Result from medical procedures.

• Superinfections: Result from antimicrobial drugs inhibiting some microbiota, allowing others to thrive.

Public Health and Disease Control

Public health agencies (e.g., CDC, WHO) monitor and control disease transmission through:

• Enforcing cleanliness of water and food

• Reducing disease vectors and reservoirs

• Immunization schedules

• Isolation and quarantine measures