Ch 14 study guide

Infection, Infectious Disease, Epidemiology

Types of Symbiosis

Symbiosis refers to the close and long-term biological interaction between two different biological organisms. The types of symbiosis include:

• Mutualism: Both organisms benefit from the relationship.

• Commensalism: One organism benefits, while the other is neither helped nor harmed.

• Parasitism: One organism (the parasite) benefits at the expense of the other (the host).

Example: Escherichia coli in the human gut (mutualism), ticks on mammals (parasitism).

Normal Microbiota and Host Relationships

Normal microbiota are microorganisms that reside in or on the human body without causing disease under normal conditions. They play important roles in health and disease.

• Resident microbiota: Microbes that are always present on or in the body.

• Transient microbiota: Microbes that are present temporarily.

• Opportunistic pathogens: Normal microbiota that can cause disease under certain conditions (e.g., immunosuppression).

Example: Staphylococcus aureus can be part of normal skin flora but may cause infection if it enters wounds.

Conditions for Opportunistic Infections

Opportunistic infections occur when normal microbiota take advantage of changes in the host environment, such as:

• Immune suppression

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

• Introduction of microbes into unusual body sites

Reservoirs of Infection

A reservoir is any person, animal, plant, soil, or substance in which an infectious agent normally lives and multiplies.

• Human reservoirs: Carriers of disease (symptomatic or asymptomatic)

• Animal reservoirs: Zoonoses (diseases transmitted from animals to humans)

• Nonliving reservoirs: Soil, water, and inanimate objects

Transmission of Pathogens

Pathogens can be transmitted through various routes:

• Direct contact: Physical interaction between hosts

• Indirect contact: Via fomites (inanimate objects)

• Droplet transmission: Respiratory droplets

• Vehicle transmission: Through air, water, or food

• Vector transmission: Via animals (e.g., insects)

Portals of Entry and Exit

Portals of entry are sites through which pathogens enter the body, such as:

• Skin

• Mucous membranes (respiratory, gastrointestinal, urogenital tracts)

• Placenta

• Parenteral route (injection, cuts)

Portals of exit are sites where pathogens leave the host, often the same as entry sites.

Virulence Factors

Virulence factors are molecules produced by pathogens that contribute to their ability to cause disease.

• Adhesion factors: Allow pathogens to attach to host cells

• Enzymes: Such as coagulase, kinase, hyaluronidase, and collagenase

• Toxins: Endotoxins (lipopolysaccharide) and exotoxins (proteins)

• Antiphagocytic factors: Capsules, biofilms, and other mechanisms to evade immune response

Stages of Infectious Disease

The progression of infectious disease typically follows these stages:

• Incubation period: Time between exposure and appearance of symptoms

• Prodromal period: Early, mild symptoms

• Illness period: Most severe symptoms

• Decline period: Symptoms subside

• Convalescence: Recovery and return to health

Classification of Infectious Diseases

Diseases can be classified based on their occurrence and spread:

• Acute: Rapid onset, short duration

• Chronic: Slow onset, long duration

• Latent: Pathogen remains inactive for a period

• Sporadic: Occurs occasionally

• Endemic: Constantly present in a population

• Pandemic: Worldwide epidemic

• Epidemic: Sudden increase in cases in a region

Contagious vs. Communicable vs. Noncommunicable Diseases

Type	Description	Example
Contagious	Easily spread from person to person	Influenza
Communicable	Can be transmitted from one host to another	HIV/AIDS
Noncommunicable	Not spread from host to host	Tetanus

Incidence and Prevalence

These are key measures in epidemiology:

• Incidence: Number of new cases in a population during a specific time period

• Prevalence: Total number of cases (new and existing) in a population at a given time

Formulas:

• Incidence rate: $\text{Incidence Rate} = \frac{\text{Number of new cases}}{\text{Population at risk} \times \text{Time period}}$

• Prevalence rate: $\text{Prevalence Rate} = \frac{\text{Total number of cases}}{\text{Total population}}$

Public Health and Disease Prevention

Public health agencies play a crucial role in controlling the spread of infectious diseases by:

• Surveillance and reporting

• Vaccination programs

• Education and awareness campaigns

• Quarantine and isolation measures

Signs and Symptoms of Disease

Signs are objective evidence of disease observable by others (e.g., fever, rash), while symptoms are subjective experiences reported by the patient (e.g., pain, fatigue).

Summary Table: Disease Occurrence Terms

Term	Definition	Example
Sporadic	Occurs occasionally and irregularly	Typhoid fever in the U.S.
Endemic	Constantly present in a population	Malaria in parts of Africa
Epidemic	Sudden increase in cases in a region	Measles outbreak
Pandemic	Worldwide epidemic	COVID-19

Additional info: Academic context and definitions have been expanded for clarity and completeness.