Infectious Disease

Key Vocabulary

Symbiotic: Interaction between two different organisms living in close physical proximity.
Commensalism: A symbiotic relationship where one organism benefits while the other is unaffected.
Mutualism: A symbiotic relationship where both organisms benefit.
Parasitism: A relationship where one organism benefits at the expense of another.
Microbiome: The collection of microorganisms living in a particular environment, especially the gut.
Resident microbiota: Microorganisms that are always present in a specific environment.
Transient microbiota: Microorganisms that are temporarily found in a given environment.
Opportunistic pathogen: Microorganisms that cause disease when the host's immune system is weakened.
Contamination: The presence of unwanted organisms in a given environment.
Infection: Invasion of a host by pathogenic microorganisms that multiply therein.
Disease: A condition resulting from infection characterized by specific signs and symptoms.
Symptoms: Subjective evidence of disease as sensed by the patient.
Signs: Objective evidence of disease as observed by a physician.
Reservoir: Natural host or habitat of a pathogenic organism.
Zoonoses: Diseases that can be transmitted from animals to humans.
Carrier: An individual who harbors a pathogen without showing symptoms.
Transmission: The way an infectious disease is spread to a new host.
Healthcare-associated infection: Infections acquired during hospital or healthcare settings.
Superinfection: Infections that occur on top of an existing infection, often due to antibiotic resistance.
Epidemiology: The study of how diseases affect the health and illness of populations.
Etiology: The study of the cause of disease.
Pathogenicity: The ability of a microorganism to cause disease.
Virulence: The degree of pathogenicity of a microorganism.

Establishment of the Microbiome

The microbiome is established at birth, influenced by the mode of delivery (vaginal vs. cesarean) and environmental exposure.
Factors affecting microbiome development include diet, age, medication (like antibiotics), and lifestyle.

Comparison of Microbiota Across Body Regions

Respiratory Tract: Rich in diverse microorganisms; typically includes Streptococcus and Haemophilus species.
Digestive Tract: Contains the highest abundance of microbiota, mainly Bacteroidetes and Firmicutes.
Urogenital Tract: Varies significantly between males and females; Lactobacillus species predominately found in females.
Skin/Eyes: Host to Staphylococcus and Corynebacterium; generally lower diversity compared to the gut.

How Normal Microbiota Can Cause Disease

Normal microbiota can become opportunistic pathogens when the host's immune defenses are compromised or when they invade sterile sites.
Examples include E. coli from the gut causing urinary tract infections or skin flora leading to surgical site infections.

Contamination, Infection, and Disease Relationship

Contamination: Presence of pathogens does not always lead to infection; the immune system can clear contaminants.
Infection: Often does not lead to disease; asymptomatic infections can occur where pathogens multiply without symptoms.

Stages of the Infectious Disease Process

Incubation: Time between pathogen entry and the appearance of first symptoms.
Prodromal: Early signs and symptoms appear; often nonspecific.
Illness: Most severe symptoms occur; pathogenesis is at its peak.
Decline: Signs and symptoms subside as the immune response or treatment takes effect.
Convalescence: Recovery period where the body repairs itself and returns to normal.

Reservoirs of Infectious Disease Agents

Animals: Domestic and wild animals that carry pathogens (e.g., rabies in bats).
Nonliving: Environmental sources, such as soil and water.
Carriers: Human hosts who harbor pathogens without showing symptoms (e.g., Typhoid Mary).

Portals of Entry and Exit for Infectious Agents

Entry: Common portals include mucous membranes (respiratory, gastrointestinal), skin, and parenteral routes (injections).
Exit: Common exit routes include respiratory secretions, feces, blood, and bodily fluids.

Identifying Mode of Transmission

Contact Transmission: Direct or indirect contact, including physical touch.
Vehicle Transmission: Transmission through contaminated water, food, or air.
Bodily Fluid Transmission: Via blood, saliva, sexual contact, etc.
Vector Transmission: Involves living organisms like insects transmitting pathogens (e.g., malaria from mosquitoes).

Sources of Pathogens in Healthcare Settings

Exogenous HA infections: Pathogens acquired from the environment or personnel.
Endogenous HA infections: Pathogens that already exist in the patient’s body causing disease.
Iatrogenic infections: Results from medical procedures or treatments.

Reducing Healthcare-Associated Infections

The number one way to reduce HA infections is through proper hand hygiene practices.

Tracking Disease in Epidemiology

Frequency: Counting occurrences of disease.
Geography: Mapping the distribution of diseases across locations.
Pattern: Understanding the trends and changes in disease occurrence over time.

Types of Epidemiological Studies

Experimental Studies: Tests hypotheses in controlled environments.
Descriptive Studies: Provide data about the who, what, where, when of outbreak.
Analytical Studies: Investigate the cause-and-effect relationships, often comparing affected individuals with those not affected.

Pathogen Adherence Factors

Infectious agents adhere to host cells via surface structures (like adhesins), which can enhance their virulence by allowing efficient colonization.

Compounds Secreted by Infectious Agents

Pathogens may secrete enzymes (e.g., proteases) and toxins (e.g., exotoxins), which can damage host tissues and enhance virulence.

Antiphagocytic Factors and Virulence

Infectious agents may employ antiphagocytic factors (e.g., capsules) that protect them from being engulfed by phagocytes, thereby boosting their ability to cause disease.