Chapter 14: Infection, Infectious Diseases, and Epidemiology – Study Notes

Symbiotic Relationships Between Microorganisms and Hosts

Types of Symbiosis

Symbiosis describes the close relationship between microorganisms and their host, which can take several forms:

• Mutualism: Both organisms benefit. Example: Microbial flora in the human colon help digest polysaccharides.

• Commensalism: One organism benefits, the other is unaffected. Example: Staphylococcus epidermidis lives on human skin without affecting the host.

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

• Amensalism: One organism is harmed, the other is unaffected. Example: Fungus Penicillium inhibits nearby bacteria.

Summary Table:

Type	Organism 1	Organism 2	Example
Mutualism	Benefits	Benefits	Bacteria in human colon
Commensalism	Benefits	Unaffected	Staphylococcus epidermidis on skin
Parasitism	Benefits	Harmed	Mycobacterium tuberculosis in lung
Amensalism	Harmed	Unaffected	Penicillium fungus inhibits bacteria

Normal Microbiota

Definition and Types

Normal microbiota (also called normal flora or indigenous microbiota) refers to the microorganisms that colonize the body's surfaces without normally causing disease.

• Axenic environments: Areas of the body free of microorganisms (e.g., womb).

• Resident microbiota: Permanent, mostly commensal, beneficial to the host.

• Transient microbiota: Temporary, cannot persist due to competition or elimination.

Locations of Resident Microbiota

• Skin

• Digestive system

• Upper respiratory tract

• Distal urethra

• Vagina

Most resident microbiota are commensal and persist throughout life.

Acquisition of Normal Microbiota

• Development in the womb is generally axenic.

• Microbiota are introduced during the birth process.

• Resident microbiota are established during the first months of life via breathing, food, and contact.

Conditions Providing Opportunities for Disease

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

  • Immune suppression (malnutrition, poor nutrition, radiation, cancer, age, stress)

  • Changes in normal microbiota (antibiotic use, microbial competition)

  • Stressful conditions (hormonal changes, emotional stress, abrupt diet changes)

• Examples: Candida albicans (yeast) and Clostridium difficile (bacterium) may overgrow when normal microbiota are disrupted.

Reservoirs of Infectious Diseases

Types of Reservoirs

Reservoirs are sites that provide a source of pathogens. There are three main types:

• Animal reservoirs: Animals (e.g., bats, rodents) can carry and spread diseases (zoonoses) without symptoms. Transmission routes include direct contact, eating animals, and vectors (e.g., mosquitoes).

• Human carriers: Infected individuals who do not show symptoms but can transmit pathogens (asymptomatic but infective).

• Nonliving reservoirs: Soil, water, and food can harbor pathogens (e.g., Clostridium botulinum in soil, contaminated water, foodborne bacteria).

Common Zoonoses Table

Disease	Causative Agent	Animal Reservoir	Mode of Transmission
Rabies	Rabies virus	Dogs	Bite of infected animal
Malaria	Plasmodium spp.	Monkeys	Bite of Anopheles mosquito
Ringworm	Microsporum	Domestic animals	Direct contact
Anthrax	Bacillus anthracis	Domestic livestock	Direct contact, inhalation
Toxoplasmosis	Toxoplasma gondii	Cats	Ingestion of contaminated food
Salmonellosis	Salmonella spp.	Poultry, reptiles	Ingestion of contaminated food

The Movement of Microbes Into Hosts

Exposure and Infection

• Exposure: Contamination or presence of microbes.

• Infection: Microbes multiply and evade host defenses. Not all infections lead to disease.

Portals of Entry

• Skin: Usually acts as a barrier; pathogens enter through breaks, cuts, or natural openings.

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

• Placenta: In 2% of pregnancies, pathogens cross the placenta to infect the fetus.

• Parenteral route: Microbes deposited directly into tissues beneath the skin (e.g., needles, cuts, surgery); not a true portal but a bypass.

Pathogens Crossing the Placenta Table

Pathogen	Condition in Adult	Effect on Embryo or Fetus
Toxoplasma gondii	Toxoplasmosis	Abortion, epilepsy, mental retardation
Treponema pallidum	Syphilis	Abortion, birth defects
Cytomegalovirus	Usually asymptomatic	Deafness, microcephaly
HIV	AIDS	Immunosuppression
Rubella virus	German measles	Severe birth defects or death

The Role of Adhesion in Infection

Adhesion Factors

• Microorganisms attach to host cells using adhesion factors or specialized structures (hooks, suckers).

• Attachment proteins (surface lipoproteins or glycoproteins) bind host cell receptors, determining specificity.

• Found on viruses and many bacteria (fimbriae, flagella, glycocalyces).

• Blockage of adhesion can prevent infection.

The Nature of Infectious Disease

Definitions

• Infection: Invasion of the host by a pathogen.

• Disease: Results only if the pathogen significantly alters normal body functions (any change from health).

• Morbidity: Another term for disease (any change with state of health).

Manifestations of Disease

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

• Signs: Observable by others (e.g., fever).

• Syndrome: Group of symptoms/signs (e.g., AIDS).

• Asymptomatic/subclinical: Infections lack symptoms but may still have signs.

Signs and Symptoms Table

Infection	Signs and Symptoms
Pneumonia	Cough, fever, chills, nausea, chest pain, shortness of breath
Meningitis	Severe headache, confusion, stiff neck, fever
Rash	Skin rash
UTIs	Pain/burning during urination, cloudy urine, lower back pain
Wound infections	Redness, swelling, tenderness, fluid drainage
Fungal infections	Fever, headache, cough, chest pain

Causation of Disease: Etiology

Etiology and Germ Theory

• Etiology: Study of the cause of disease.

• Germ theory of disease: Diseases are caused by infections of pathogenic microorganisms.

Koch's Postulates

1. Suspected causative agent must be found in every case of the disease and be absent from healthy hosts.

2. Agent must be isolated and grown outside the host.

3. When agent is introduced into a healthy, susceptible host, the host must get the disease.

4. Same agent must be re-isolated from diseased experimental host.

Virulence Factors of Infectious Agents

Definitions

• Pathogenicity: Ability of a microorganism to cause disease.

• Virulence: Degree of pathogenicity.

• Virulence factors: Traits that contribute to virulence, not found in nonvirulent microbes.

Examples of Virulence Factors

• Extracellular enzymes

• Toxins

• Antiphagocytic factors

Extracellular Enzymes

• Enzymes such as hyaluronidase and collagenase help pathogens invade tissues.

• Coagulase causes blood clotting, providing a hiding place for bacteria.

• Kinases (e.g., staphylokinase, streptokinase) digest blood clots, aiding spread.

Toxins

• Chemicals produced by pathogens to harm tissues or trigger immune responses.

• Toxemia: Toxins enter bloodstream and are carried beyond infection site.

• Two types:

  • Exotoxins: Secreted into environment; include cytotoxins (kill host cells), neurotoxins (interfere with nerve cells), enterotoxins (affect GI tract).

  • Endotoxins: Lipid A, part of LPS layer in gram-negative bacteria; released from dead/dying bacteria, can cause systemic effects (e.g., E. coli outbreak).

• Body protects itself with antitoxins (antibodies that bind and destroy toxins).

Antiphagocytic Factors

• Prevent pathogen from being phagocytosed by host's immune cells.

• Bacterial capsule: Composed of host-like chemicals, not recognized as foreign; slippery, hard to engulf.

• Antiphagocytic chemicals: Prevent fusion of lysosome and phagocytic vesicles; leucocidins destroy phagocytic white blood cells.

The Stages of Infectious Disease

Stages

• Incubation period: Time between infection and first symptoms.

• Prodromal period: Mild, generalized symptoms.

• Illness: Most severe stage, signs and symptoms most evident.

• Decline: Immune response and treatments reduce pathogen.

• Convalescence: Recovery and tissue repair.

Not every disease has every stage; a patient may be infectious during any stage.

Incubation Periods Table

Disease	Incubation Period
Staphylococcal 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	75 to 100 days
AIDS	1 to 8 years
Leprosy	18 to >30 years

Movement of Pathogen Out of Host: Portals of Exit

• Pathogens leave the host through portals of exit, including:

  • Bodily secretions (tears, earwax, nasal secretions, saliva, sputum, respiratory droplets)

  • Blood (arthropod bites, hypodermic needles, wounds)

  • Bodily wastes (feces, urine)

  • Some portals of entry

Modes of Infectious Disease Transmission

Types of Transmission

• Contact transmission: Direct, indirect (needles, tissues, toys, coins), or droplet (coughing, sneezing).

• Vehicle transmission: Airborne (aerosols), waterborne (fecal-oral), foodborne.

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

Communicable vs. Noncommunicable Diseases

• Communicable: Transmitted from another diseased host (e.g., influenza, herpes, TB).

• Contagious: Easily transmitted communicable disease (e.g., measles, smallpox).

• Noncommunicable: Not transmitted from host to host (e.g., tetanus from soil).

Longevity and Severity of Disease

• Acute: Develops rapidly, short duration (e.g., cold).

• Chronic: Develops slowly, persists (e.g., tuberculosis).

• Subacute: Duration/severity between acute and chronic.

• Latent: Pathogen remains inactive for a time before symptoms appear (e.g., herpes, HIV).

Epidemiology of Infectious Diseases

Definitions and Measures

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

• Incidence: Number of new cases / number of people at risk

• Prevalence: Number of old and new cases / total number of people at risk

Distribution of Disease

• Endemic: Occurs at a stable frequency in a population or region.

• Epidemic: Occurs at a higher frequency than usual.

• Pandemic: Epidemic on more than one continent.

Example: Influenza is endemic, but outbreaks can lead to epidemics or pandemics (e.g., H1N1 in 2009).

Approaches of Epidemiology

• Descriptive epidemiology: Collects and analyzes data to identify the index case (first case).

• Analytical epidemiology: Analyzes data to determine probable cause, mode of transmission, and prevention methods.

• Experimental epidemiology: Tests hypotheses concerning the cause of disease, often using Koch's postulates.

Nosocomial (Healthcare-Associated) Infections

• Exogenous: Acquired from the healthcare environment (e.g., air conditioning, bed rails).

• Endogenous: Arise from normal microbiota due to hospitalization or treatment (e.g., chemotherapy, superinfection).

• Iatrogenic: Direct result of medical procedures (e.g., surgery).

Prevention includes disinfection, medical asepsis, good hygiene, and infection control protocols.

Public Health and Epidemiology Agencies

• Agencies at local, state, national, and global levels share information about diseases.

• United States Public Health Service (USPHS) and Centers for Disease Control and Prevention (CDC) collect statistics, research, and make recommendations.

• CDC publishes the Morbidity and Mortality Weekly Report (MMWR).

• International organizations (e.g., ProMED, IDSA) track and alert for infectious diseases.