Infection, Infectious Diseases, and Epidemiology Study Guide

Symbiotic Relationships Between Microbes and Their Hosts

Definition of Symbiosis: The term symbiosis literally means "to live together." This relationship exists between humans and countless microorganisms.
Types of Symbiotic Relationships (Table 14.1): * Mutualism: A relationship where both Organism 1 and Organism 2 benefit. An example is bacteria living in the human colon. * Commensalism: A relationship where Organism 1 benefits, while Organism 2 neither benefits nor is harmed. An example is $Staphylococcus$ living on the skin. Absolute commensalism is often difficult to prove because a subtle benefit or harm may exist that is not easily observed. * Parasitism: A relationship where Organism 1 benefits and Organism 2 is harmed. An example is $Tuberculosis$ bacteria in the human lung.
Pathogens: Any parasite that is capable of causing disease is referred to as a pathogen.

Normal Microbiota in Hosts

Definition: Organisms that colonize the surfaces of the body without normally causing disease are called normal microbiota. They are also known as normal flora or indigenous microbiota.
Types of Normal Microbiota: 1. Resident Microbiota: These are a permanent part of the normal microbiota throughout a person's life. Most resident microbiota are commensal. 2. Transient Microbiota: Found in the same regions as resident microbiota but remain in the body for only a short period. They cannot persist for three primary reasons: * Competition from other microorganisms. * Elimination by the body's defense cells (immune system). * Chemical or physical changes within the body.
Acquisition of Normal Microbiota: * Axenic Environment: While a fetus is developing in the womb, it is generally free of microorganisms, a state known as being axenic. * Initial Colonization: Microbiota begin to develop during the birthing process. * Establishment: Most of a person's resident microbiota is established during the first few months of life.

Opportunistic Pathogens

Definition: Opportunistic pathogens are normal microbiota that cause disease only under certain circumstances (Concept 14.6).
Conditions for Opportunism (Concept 14.7): 1. Introduction into unusual sites: Movement of normal microbiota to a part of the body where they do not normally reside (e.g., $E. coli$ entering the urinary tract). 2. Immune suppression: Weakening of the immune system (e.g., by AIDS or other factors). 3. Changes in relative abundance: A decrease in microbial antagonism or competition (normal flora competing with each other) can allow one member of the microbiota to thrive and cause disease.

Reservoirs of Infectious Diseases

Definition: Reservoirs of infection are sites where pathogens are maintained as a source of infection. Most pathogens cannot survive for long periods outside of their host.
Types of Reservoirs: 1. Animal Reservoirs (Zoonoses): Diseases naturally spread from animal hosts to humans. Acquisition routes include: * Direct contact with the animal or its waste. * Eating animals (e.g., the connection between fruit bats and Ebola). * Bloodsucking arthropods. * Example: Rabies virus moving from bats/foxes/skunks to pets, then to humans. 2. Human Carriers: * Asymptomatic carriers: Infected individuals who show no symptoms but can remain infective to others. * Healthy carriers: May have defensive systems that protect them from illness while they harbor the pathogen. 3. Nonliving Reservoirs: Soil, water, and food. The presence of microorganisms in these reservoirs is often due to contamination by feces or urine.

Exposure and Entry of Microbes

Key Definitions: * Contamination: The mere presence of microbes in or on the body. * Infection: Occurs when an organism evades the body's external defenses, multiplies, and becomes established. Not every contamination results in an infection. * Disease (Morbidity): Results only if the invading pathogen alters normal body functions.
Portals of Entry (Concept 14.15): 1. Skin: The outer layer of dead skin cells acts as a barrier. Pathogens may enter through openings, cuts, or by burrowing into/digesting outer layers. 2. Mucous Membranes: Line body cavities open to the environment. They provide a moist, warm environment. * Respiratory tract: The most common site of entry (nose, mouth, eyes). Examples include cold and influenza viruses. * Gastrointestinal tract: Pathogens must survive the acidic pH of the stomach to enter here. 3. Placenta: Usually forms an effective barrier, but some pathogens can cross it, leading to spontaneous abortion, birth defects, or premature birth. 4. Parenteral Route: Not a true portal, but a means by which portals are circumvented (e.g., punctures by nails, thorns, needles, cuts, bites, or surgery).

Adhesion in Infection

Definition: Adhesion is the process by which microorganisms attach themselves to cells, which is required to establish colonies successfully.
Adhesion Factors: * Specialized structures: Fimbriae and glycocalyx. * Attachment proteins: Found on viruses and many bacteria. These ligands (viral or bacterial) bind to host cell receptors. * Clinical Importance: Changing or blocking a ligand or its receptor can prevent infection by disrupting host cell specificity.

The Nature of Infectious Disease: Manifestations

Terminology: * Symptoms: Subjective characteristics felt only by the patient (e.g., pain, nausea, headache, chills, sore throat, fatigue/lethargy, malaise, itching, abdominal cramps). * Signs: Objective manifestations observed or measured by others (e.g., swelling, rash, redness, vomiting, diarrhea, fever, pus formation, anemia, bubo/swollen lymph node). * Syndrome: A group of symptoms and signs that characterize a specific disease or abnormal condition. * Asymptomatic (Subclinical): Infections that lack symptoms but may still show signs (e.g., abnormal blood counts).
Vital Signs and Medical Terms: * Leukocytosis/Leukopenia: Increase/decrease in the number of circulating white blood cells. * Tachycardia/Bradycardia: Increase/decrease in heart rate.

Etiology and Koch’s Postulates

Etiology: The study of the cause of disease.
Robert Koch's Postulates: A set of conditions that must be satisfied to prove a particular pathogen causes a specific disease.
Exceptions to Koch’s Postulates: * Pathogens that cannot be cultured in a laboratory. * Diseases caused by a combination of pathogens and other cofactors. * Ethical considerations that prevent applying postulates to pathogens requiring a human host.

Virulence Factors of Infectious Agents

Pathogenicity vs. Virulence: * Pathogenicity: The ability of a microorganism to cause disease. * Virulence: The ease with which the organism can cause disease (degree of pathogenicity). It is not necessarily about the severity of the disease.
Extracellular Enzymes: Secreted by pathogens to dissolve structural chemicals in the body, helping them invade and avoid defenses.
Toxins: Chemicals that harm tissues or trigger damaging immune responses. * Toxemia: Presence of toxins in the bloodstream, allowing them to be carried beyond the site of infection.
Comparison of Exotoxins and Endotoxins (Table 14.8): * Exotoxins: * Source: Mainly Gram-positive and Gram-negative bacteria. * Relation: Metabolic product secreted from a living cell. * Nature: Protein or short peptide. * Toxicity: High. * Heat Stability: Unstable at temperatures above $60^\circ C$ . * Antigenicity: Strong (stimulates antitoxin/antibody production); toxoids can be used for immunization. * Examples: Botulism, tetanus, diphtheria, cholera. * Endotoxins: * Source: Gram-negative bacteria only. * Relation: Portion of the outer cell wall membrane (Lipid A) released upon cell death. * Nature: Lipid portion of lipopolysaccharide (LPS). * Toxicity: Low, but can be fatal in high doses. * Heat Stability: Stable for up to $1\,hour$ at autoclave temperature ( $121^\circ C$ ). * Effect: Fever, lethargy, malaise, shock, blood coagulation. * Examples: Typhoid fever, endotoxic shock, urinary tract infections.
Antiphagocytic Factors: * Bacterial Capsules: Composed of chemicals (like hyaluronic acid) that are not recognized as foreign and are physically slippery, making them hard to engulf. * Antiphagocytic Chemicals: Some prevent the fusion of lysosomes with phagocytic vesicles. Leukocidins directly destroy phagocytic white blood cells.

Stages of Infectious Disease

The disease process follows five stages (Acronym: IPIDC):

Incubation Period: Time between infection and first symptoms.
Prodromal Period: Short timeframe of generalized, mild symptoms.
Illness: Most severe stage; signs and symptoms are most evident.
Decline: Body returns to normal as immune response or treatment overcomes the pathogen.
Convalescence: Patient recovers from the illness; tissues are repaired.

Transmission of Infectious Diseases

Portals of Exit: Pathogens often leave the host in secretes or excretes (often the same as portals of entry).
Transmission Groups: 1. Contact Transmission: * Direct: Handshaking, kissing, sexual intercourse, bites. * Indirect: Spread via fomites (inanimate objects like needles, toothbrushes, drinking glasses). * Droplet: Spread in mucus droplets via exhaling, coughing, or sneezing within $1\,meter$ . 2. Vehicle Transmission: * Airborne: Travels more than $1\,meter$ via aerosol (dust, sneezing, air-conditioning). * Waterborne: Important for gastrointestinal diseases (fecal-oral infection). * Foodborne: Inadequately processed, cooked, or refrigerated food. * Bodily Fluid Transmission: Blood, urine, and saliva entering through conjunctiva or breaks in skin. 3. Vector Transmission: * Biological Vectors: Arthropods (mosquitoes, ticks) that serve as hosts for a stage of the pathogen's life cycle (e.g., Malaria, Lyme disease). * Mechanical Vectors: Passively carry pathogens on their bodies (e.g., houseflies and cockroaches carrying $E. coli$ or $Salmonella$ ).

Classification of Diseases

Longevity and Severity (Table 14.12): * Acute: Symptoms develop rapidly; runs course quickly. * Chronic: Mild symptoms develop slowly and last a long time. * Subacute: Between acute and chronic. * Latent: Disease appears a long time after infection.
Other Classifications: * Communicable: Transmitted from one host to another. * Contagious: A communicable disease that is easily spread. * Noncommunicable: Arising from outside hosts or opportunistic pathogens. * Local Infection: Confined to a small region. * Systemic Infection: Widespread infection in many systems, often traveling in blood or lymph. * Focal Infection: Serves as a source of pathogens for infections at other sites. * Primary vs. Secondary: Initial infection vs. follow-up infection by an opportunistic pathogen.

Epidemiology

Definition: The study of where and when diseases occur and how they are transmitted in populations.
Measures of Disease Occurrence: 1. Incidence: Number of new cases in a given area/period. 2. Prevalence: Number of total cases (old and new). Prevalence is always larger than incidence.
Frequency Terminology: * Endemic: Disease occurs normally at a stable rate within a population. * Sporadic: Only a few scattered cases occur. * Epidemic: Disease occurs at a greater frequency than normal for an area or population. * Pandemic: An epidemic occurring simultaneously on more than one continent.
Epidemiological Study Methods: * Descriptive Epidemiology: Careful tabulation of data (location, time, patient info). Focuses on identifying the index case. * Analytical Epidemiology: Determines probable cause, mode of transmission, and prevention methods. * Experimental Epidemiology: Testing hypotheses regarding disease causes using Koch's postulates.

Hospital Epidemiology: Nosocomial Infections

Definition: Infections acquired in a health care setting.
Types of Nosocomial Infections: 1. Exogenous: Pathogen acquired from the health care environment. 2. Endogenous: Pathogen arises from normal microbiota within the patient due to factors in the health care setting. 3. Iatrogenic: Resulting from modern medical procedures/techniques (catheters, surgery). 4. Superinfections: Use of antimicrobials reduces competition from resident microbiota, allowing others to thrive.
Control/Prevention: Aggressive measures are required. Handwashing is established as the most effective way to reduce nosocomial infections.

Public Health

Agencies: * National: United States Public Health Service; Centers for Disease Control and Prevention (CDC). * Global: World Health Organization (WHO).
Public Health Functions: * Monitor water and food safety. * Reduce disease vectors and reservoirs. * Enforce immunization schedules. * Locate and treat individuals exposed to contagious pathogens. * Establish isolation and quarantine measures. * MMWR: Public health data is reported in the CDC's Morbidity and Mortality Weekly Report.