Infection, Infectious Diseases, and Epidemiology

Symbiotic Relationships Between Microbes and Their Hosts

Microorganisms interact with their hosts in various ways, forming symbiotic relationships that can impact health and disease. Symbiosis refers to the close association between two different species living together.

• Mutualism: Both organisms benefit from the relationship. Example: Bacteria in the human colon synthesize vitamins and receive nutrients.

• Commensalism: One organism benefits, while the other is neither helped nor harmed. Example: Mites in human hair follicles.

• Amensalism: One organism is harmed, while the other is unaffected. Example: Fungi secreting antibiotics that inhibit nearby bacteria.

• Parasitism: One organism benefits at the expense of the other. Example: Mycobacterium tuberculosis in human lungs.

Microbiome of Humans

The human microbiome consists of all microorganisms that colonize the body's surfaces without causing disease under normal conditions. These are also called normal microbiota, normal flora, or indigenous microbiota.

• Resident microbiota: Remain throughout life, mostly commensal.

• Transient microbiota: Present for short periods; cannot persist due to competition, immune responses, or environmental changes.

Major sites of normal microbiota include the skin, eyes, upper respiratory tract, and digestive tract.

Acquisition and Changes in the Microbiome

Humans are born without microbiota; colonization begins during birth and continues in the first months of life. The composition of the microbiome can change due to age, diet, environment, and health status.

Opportunistic Pathogens

Normal microbiota can become opportunistic pathogens under certain conditions, such as:

• Introduction into unusual body sites

• Immune suppression

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

• Stressful conditions

Reservoirs of Infectious Diseases of Humans

Types of Reservoirs

Reservoirs are sites where pathogens are maintained as sources of infection. There are three main types:

• Animal reservoirs: Pathogens that infect animals and can be transmitted to humans (zoonoses).

• Human carriers: Infected individuals who may not show symptoms but can transmit pathogens.

• Nonliving reservoirs: Soil, water, and food contaminated with infectious agents.

Zoonoses

Zoonoses are diseases that naturally spread from animal hosts to humans. Transmission can occur through direct contact, consumption, or vectors such as arthropods. Humans are often dead-end hosts for these pathogens.

The Invasion and Establishment of Microbes in Hosts: Infection

Contamination vs. Infection

Contamination is the mere presence of microbes on or in the body, while infection occurs when microbes evade external defenses, multiply, and establish themselves in the host.

Stages of Infectious Diseases

Infectious diseases typically progress through several stages:

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

• Prodromal period: Mild, general symptoms appear.

• Illness: Most severe signs and symptoms.

• Decline: Symptoms subside as immune response or treatment reduces pathogen numbers.

• Convalescence: Recovery; no signs or symptoms remain.

Incubation Periods of Selected Infectious Diseases

The Movement of Pathogens Out of Hosts: Portals of Exit

Portals of Exit

Pathogens leave the host through specific portals of exit, often the same as portals of entry. These include secretions, excretions, and other bodily fluids.

Modes of Infectious Disease Transmission

Overview of Transmission

Transmission is the movement of pathogens from a reservoir or portal of exit to another host's portal of entry. There are three main groups:

• Contact transmission

• Vehicle transmission

• Vector transmission

Contact Transmission

• Direct contact: Physical interaction between hosts (e.g., touching, kissing, sexual contact).

• Indirect contact: Transmission via fomites (inanimate objects).

• Droplet transmission: Spread via droplets of mucus from coughing, sneezing, or talking.

Vehicle Transmission

• Airborne transmission: Pathogens travel more than 1 meter via aerosols (e.g., dust, droplets).

• Waterborne transmission: Pathogens spread through contaminated water, often causing gastrointestinal diseases.

• Foodborne transmission: Pathogens in or on foods, especially if improperly processed or stored.

• Bodily fluid transmission: Pathogens in blood, urine, saliva, etc., can infect if they contact mucous membranes or broken skin.

Vector Transmission

• Biological vectors: Arthropods that transmit pathogens and serve as hosts for part of the pathogen's life cycle (e.g., mosquitoes, ticks).

• Mechanical vectors: Arthropods that passively carry pathogens on their bodies (e.g., houseflies, cockroaches).

Summary Table: Modes of Disease Transmission

Additional info: Understanding these mechanisms is essential for controlling infectious diseases and preventing outbreaks in both clinical and public health settings.