Infectious and Communicable Diseases Study Guide

Overview of Infectious and Communicable Diseases

• Conceptual Definitions

  • Infectious Disease: The presence and replication of an infectious agent in the tissues of a host, accompanied by the manifestation of signs and symptoms.

  • Communicable (Contagious) Disease: To be classified as communicable, an infectious disease must have a portal of exit from the infected person (or animal/insect), a means of transmission, and a portal of entry to a susceptible host.

• The Epidemiological Triangle

  • Agent (The "What"): The microbe that causes the disease.

  • Host (The "Who"): The human, animal, or insect that is harboring the infectious agent.

  • Environment (The "Where"): External factors that cause or allow the transmission of the disease.

• Properties of Infectious Agents

  • Pathogenicity: The ability of an infectious agent to cause damage or produce an infectious disease in a susceptible host.

  • Infectivity: The ability of an infectious agent to invade the host and replicate.

  • Virulence: The severity of damage caused by the infectious agent.

  • Toxicity: The ability of the agent to produce toxins.

  • Immunogenicity: The ability of the agent or antigen to produce specific immunity within the host.

  • Invasiveness: The ability of the agent to destroy healthy tissue.

• Agent Characteristics

  • Factors affecting the cycle of infection include the agent's:

    • Size

    • Shape

    • Chemical composition

    • Growth requirements

    • Viability (ability to survive for extended periods)

  • Types of Agents:

    • Bacteria

    • Viruses

    • Rickettsiae

    • Fungi

    • Protozoa

    • Helminths (parasitic worms)

Disease Progression and Timing

• Incubation Period: The time interval between exposure to an infectious agent and the manifestation of symptoms in the host. This period varies by disease and between different hosts.

• Latent Period: Occurs when the infectious agent has invaded a host and found hospitable conditions to replicate. Replication occurs during this time, but it happens before shedding begins.

• Communicable Period: This period follows latency and begins with the shedding of the agent. It may overlap with the incubation period.

• Infection States:

  • Latent Infection: The agent is present but not yet being shed.

  • Patent Infection: The agent is being shed.

Reservoirs and Portals

• Reservoirs: Any organic substance where an agent can live and multiply, including:

  • Humans

  • Animals

  • Arthropods

  • Plants

  • Soil

  • Water

• Portals of Exit:

  • Respiratory secretions

  • Vaginal secretions

  • Semen

  • Saliva

  • Exudates

  • Blood

  • Feces

• Portals of Entry:

  • Respiratory passages

  • Mucous membranes

  • Skin

  • Blood vessels

  • Oral cavity

  • Placenta

Modes of Transmission

• Direct Transmission: The physical transfer of microorganisms between a susceptible host and an infected or colonized person.

• Indirect Transmission:

  • Vectors: Animals that transmit infectious agents through biological and mechanical routes.

  • Fomites: Any inanimate objects, materials, or substances that act as transport agents for a microbe.

• Droplet Transmission: Droplets are propelled a distance of $\le 3\,\text{feet}$ through the air and deposited on the conjunctivae, oral, or nasal mucosa of a susceptible person.

• Airborne Transmission:

  • Microbial Aerosols: Droplet nuclei, such as those carrying Tuberculosis ($\text{TB}$).

  • Other Sources: Fungal spores, viruses, and bacteria.

Host Factors and Definitions

• Susceptibility: Not all humans are equally susceptible to or at risk for contracting infectious diseases.

• Carrier: A person or animal who harbors an infectious organism and transmits the organism to others while having no symptoms of the disease.

• Colonization: The presence and multiplication of microorganisms on or within a host without necessarily causing an immune response or symptoms.

Outbreak Investigation

• Definitions of Occurrence:

  • Endemic: The constant presence of a disease within a geographic area or population.

  • Epidemic: An occurrence in a community or region of cases of an illness clearly in excess of normal expectancy.

  • Pandemic: An epidemic occurring worldwide, or over a very wide area, crossing international boundaries and usually affecting a large number of people.

• CDC Steps in an Outbreak Investigation:

  1. Establish the existence of an outbreak.

  2. Verify the diagnosis.

  3. Construct a working case definition.

  4. Find cases systematically and record information.

  5. Perform descriptive epidemiology.

  6. Develop hypotheses.

  7. Evaluate hypotheses epidemiologicaly.

  8. As necessary, reconsider, refine, and reevaluate hypotheses.

  9. Compare and reconcile with laboratory and/or environmental studies.

  10. Implement control and prevention measures.

  11. Initiate or maintain surveillance.

  12. Communicate findings.

Healthcare-Associated Infections (HAIs)

• Prevalence and Impact:

  • Approximately $1 \text{ in every } 20$ inpatients has an infection related to healthcare (nosocomial infections).

  • The annual cost is measured in billions of dollars.

  • HAIs cause tens of thousands of deaths annually.

  • In Long-Term Care Facilities ($\text{LTCFs}$), an estimated $1 \text{ to } 3\,\text{million}$ serious HAIs occur every year.

  • $400,000$ people die from HAIs in these facilities annually.

• Most Common HAIs:

  • Urinary tract infections ($\text{UTIs}$)

  • Surgical-site infections ($\text{SSIs}$)

  • Bloodstream infections

  • Pneumonia

Community-Acquired and Specific Illnesses

• Common Infections:

  • Seasonal influenza (Flu A, Flu B, COVID)

  • Pneumococcal disease

  • Asymptomatic bacteriuria (Chronic $\text{UTIs}$)

  • Foodborne illnesses (e.g., $\text{Hepatitis A}$)

  • Waterborne illnesses

  • Bloodborne illnesses

  • Sexually Transmitted Infections ($\text{STIs}$)

• Foodborne Pathogens:

  • Noroviruses: Spread by fecal-oral route; common foodborne pathogens.

  • Campylobacter infection: A diarrheal illness often referred to as "traveler's diarrhea."

  • Listeria monocytogenes: Causes Listeriosis; uncommon but potentially fatal.

  • Nontyphoid Salmonella: Bacterial disease transmitted via contaminated food and water.

  • Escherichia coli O157:H7: A deadly form of $\text{E. coli}$ causing severe abdominal cramps, bloody diarrhea, and vomiting. Usually resolves in $7 \text{--} 10\,\text{days}$, though the elderly are more vulnerable.

• Waterborne Illnesses:

  • At least $2\,\text{billion}$ people globally lack access to adequate sanitation and use fecal-contaminated drinking water.

  • Diarrheal diseases like cholera kill more children than $\text{AIDS}$, malaria, and measles combined.

  • It is the second leading cause of death among children less than $5\,\text{years}$ of age.

Sexually Transmitted Infections (STIs)

• Prevalence: $20\,\%$ ($1 \text{ in } 5$) of the U.S. population has an $\text{STI}$ on any given day.

• Specific STIs:

  • Chlamydia: The most prevalent $\text{STI}$.

  • Gonorrhea: Showing an increase in cases.

  • Syphilis: Congenital syphilis cases are increasing.

  • Human Papillomavirus (HPV): Preventable via vaccination.

  • Human Immunodeficiency Virus (HIV): Management involves $\text{PrEP}$ (Pre-Exposure Prophylaxis) vs. $\text{PEP}$ (Post-Exposure Prophylaxis).

  • Herpes Simplex Virus (HSV): A lifelong infection.

  • Hepatitis viruses: Types A, B, and C.

• Patient Education:

  • Genital discharge, burning during urination, or unusual sores/rashes should signal immediate cessation of sexual activity and consultation with a healthcare provider.

  • Treatment: Bacterial infections can be treated/cured with antibiotics; viral infections cannot be cured with antibiotics.

  • Partner Notification: All partners must be notified for examination and treatment, often via $EPT$ (Expedited Partner Therapy). Resources include Letthemknow.com.

Reemerging Vaccine-Preventable Diseases

• List of Diseases:

  • Measles

  • Mumps

  • Rubella

  • Pertussis

  • Varicella

  • Polio

  • $RSV$

• Public Health Resources:

  • Livingston County Health Department ($\text{LCHD}$) provides Hybrid Immunization Waiver Education.

  • Contact: $517\text{--}546\text{--}9850$.

  • Online portal available for training at https://survey.alchemer.com/s3/7787303/LCHD-Virtual-Immunization-Waiver-Education.

Questions & Discussion

• Question: Is the following statement true or false? A carrier is a person or animal who harbors an infectious organism and transmits the organism to others while having no symptoms of the disease.

  • Response: True. (This aligns with the definition of a carrier who remains asymptomatic while infectious).

• Discussion Point on Transmission Prevention:

  • What $\text{PPE}$ will prevent transmission?

  • What are some other ways to prevent transmission?

• Discussion Point on Reservoirs:

  • Why is knowing the reservoir and portals important? Knowing the reservoir helps in identifying the source and breaking the chain of infection. Portals of exit and entry identify necessary precautions (e.g., masks for respiratory portals).