Ch 14 lecture

Infection, Infectious Disease, and Epidemiology

Introduction

This chapter explores the relationships between microbes and their hosts, the classification and stages of infectious diseases, mechanisms of disease transmission, and the principles of epidemiology. Understanding these concepts is essential for studying how diseases develop, spread, and are controlled in populations.

Relationships Between Microbes and Their Hosts

Symbiosis and Microbial Interactions

Microbes interact with their hosts in various ways, forming different types of symbiotic relationships. The term symbiosis means "to live together," and it encompasses several forms of association between organisms.

• Mutualism: Both organisms benefit from the relationship. Example: Bacteria in the human colon produce vitamins that humans absorb.

• Commensalism: One organism benefits, while the other is neither helped nor harmed. Example: Staphylococcus on human skin; bacteria benefit from nutrients, but humans are unaffected.

• Parasitism: One organism benefits at the expense of the other. Example: Tuberculosis bacteria in the human lung cause disease in the host.

These relationships can change over time, especially if the host's environment or immune status changes. For example, immunocompromised individuals are more susceptible to opportunistic infections.

Table: Types of Symbiotic Relationships

Type	Organism 1	Organism 2	Example
Mutualism	Benefits	Benefits	Bacteria in human colon
Commensalism	Benefits	Neither benefits nor is harmed	Staphylococcus on skin
Parasitism	Benefits	Is harmed	Tuberculosis bacteria in human lung

Normal Microbiota of the Human Body

Types and Distribution

The human body hosts a vast array of microbes, collectively known as the normal microbiota or flora. These microbes typically reside on body surfaces without causing disease.

• Resident microbiota: Remain in the body for long periods and are established early in life.

• Transient microbiota: Present for short periods and do not permanently colonize the host.

Microbial populations vary by body site (e.g., skin, respiratory tract, digestive tract) and can differ between individuals (e.g., men and women have different skin microbiota).

Pathogenicity and Opportunistic Pathogens

Pathogens and Disease

A pathogen is a microbe capable of causing disease. Sometimes, normal microbiota can become opportunistic pathogens under certain conditions, such as immune suppression or when introduced into unusual body sites.

• Example: Candida albicans is normally harmless but can cause fungal infections in immunocompromised hosts.

Reservoirs of Infectious Disease

Types of Reservoirs

Reservoirs are sources where pathogens are maintained and from which infection can occur.

• Animal reservoirs: Diseases maintained in animals and transmitted to humans (zoonoses).

• Human reservoirs: Infected individuals, including asymptomatic carriers.

• Nonliving reservoirs: Soil, water, and food can harbor pathogens.

Zoonoses are diseases that naturally spread from animals to humans, such as rabies and Ebola.

Portals of Entry and Establishment of Infection

How Pathogens Enter the Host

Pathogens must enter the host through specific portals of entry to cause infection.

• Skin: Acts as a barrier, but pathogens can enter through cuts, bites, or natural openings.

• Mucous membranes: Line body cavities and provide moist environments for pathogen entry (e.g., respiratory, digestive, urinary tracts).

• Placenta: Usually a barrier, but some pathogens can cross and infect the fetus.

• Parenteral route: Pathogens deposited directly beneath the skin (e.g., via injection).

Adhesion and Establishment

After entry, pathogens must adhere to host cells using specialized structures called adhesion factors (ligands).

• Ligands are proteins or glycoproteins on the pathogen surface that bind to host cell receptors.

• Some pathogens form biofilms for enhanced attachment and protection.

Manifestations and Classification of Infectious Disease

Signs, Symptoms, and Syndromes

Diseases are characterized by signs (objective, measurable changes) and symptoms (subjective experiences felt by the patient). A syndrome is a group of signs and symptoms that characterize a disease.

• Asymptomatic (subclinical) infections: Lack symptoms but may still show signs.

Koch's Postulates

Koch's postulates are criteria used to establish a causative relationship between a microbe and a disease:

1. The suspected pathogen must be present in all cases of the disease.

2. The pathogen must be isolated and grown in pure culture.

3. The cultured pathogen must cause disease when introduced into a healthy host.

4. The same pathogen must be re-isolated from the diseased experimental host.

Limitations: Some pathogens cannot be cultured, may require a human host, or diseases may be caused by multiple pathogens.

Virulence Factors

Mechanisms of Pathogenicity

Virulence is the degree to which a pathogen can cause disease. Pathogens possess various virulence factors:

• Adhesion factors: Enable attachment to host cells.

• Extracellular enzymes: Break down host tissues (e.g., collagenase, coagulase, kinase).

• Toxins: Chemicals that harm tissues or trigger damaging immune responses. Exotoxins: Secreted by bacteria, affect specific cells. Endotoxins: Released from Gram-negative bacteria upon cell death, can cause shock and fever.

• Antiphagocytic factors: Prevent phagocytosis by host immune cells (e.g., capsules).

Stages of Infectious Disease

Progression of Disease

Infectious diseases typically progress through five stages:

1. Incubation period: Time between infection and appearance of symptoms.

2. Prodromal period: Mild, general symptoms.

3. Illness: Most severe signs and symptoms; immune system has not yet responded.

4. Decline: Pathogen is being killed; symptoms subside.

5. Convalescence: Tissues recover; patient regains strength.

Transmission of Infectious Disease

Modes of Transmission

Pathogens are spread through various mechanisms:

• Contact transmission: Direct (person-to-person), indirect (fomites), or droplet (respiratory droplets).

• Vehicle transmission: Airborne, waterborne, or foodborne routes.

• Vector transmission: Animals (usually arthropods) that transmit pathogens. Biological vectors: Pathogen develops within the vector (e.g., mosquitoes). Mechanical vectors: Pathogen is passively carried (e.g., houseflies).

Classification of Infectious Diseases

By Severity and Spread

Diseases can be classified based on their duration, severity, and how they spread:

• Acute diseases: Develop rapidly, have short duration.

• Chronic diseases: Develop slowly, last longer, may recur.

• Latent diseases: Pathogen remains inactive for long periods.

• Contagious diseases: Easily spread from person to person.

Epidemiology of Infectious Disease

Incidence and Prevalence

Epidemiology is the study of where and when diseases occur and how they are transmitted. Two key measures are:

• Incidence: Number of new cases in a given area during a specific time period.

• Prevalence: Total number of cases (new and existing) in a given area during a specific time period.

These measures help track disease outbreaks and inform public health responses.

Types of Epidemiological Studies

• Descriptive studies: Collect data on location, time, and patient information to identify the index case.

• Analytical studies: Compare affected and unaffected individuals to determine causes and risk factors.

• Experimental studies: Test hypotheses about disease transmission and prevention.

Nosocomial (Healthcare-Associated) Infections

Hospital-Acquired Infections

Nosocomial infections are acquired in healthcare settings and are a major public health concern. They arise due to the presence of microorganisms, compromised patients, and transmission among staff and patients.

• Prevention: Handwashing is one of the most effective methods to reduce nosocomial infections.

Public Health and Disease Control

Role of Agencies

Local, national, and global agencies (e.g., CDC, WHO) monitor disease, promote health, and implement measures to limit disease spread, such as water and food safety campaigns and vaccination programs.

Summary Table: Key Terms in Epidemiology

Term	Definition
Incidence	Number of new cases in a population during a specific time
Prevalence	Total number of cases (new and existing) in a population during a specific time
Endemic	Disease constantly present in a population
Epidemic	Disease occurs at a higher than expected rate
Pandemic	Worldwide epidemic

Additional info: Some explanations and examples have been expanded for clarity and completeness, based on standard microbiology textbook content.