Infection, Infectious Diseases, and Epidemiology

Symbiotic Relationships

Symbiosis refers to biological interactions or relationships between two different organisms. There are three main forms:

• Parasitism: One organism (the parasite) benefits at the expense of the other (the host), which is harmed. Example: Mycobacterium tuberculosis (causes TB in human lungs)

• Mutualism: Both organisms benefit from the relationship. Example: Bacteria in the human colon

• Commensalism: One organism benefits, and the other is unaffected. Example: Mites in human hair follicles

Normal Microbiota

Microbiota are organisms that colonize the body’s surfaces without normally causing disease. They are divided into:

• Resident Microbiota: Always present, mostly commensal, part of the normal microbiota throughout life.

• Transient Microbiota: Remain in the body for a short period, found in the same regions as resident microbiota, but cannot persist due to competition, elimination, or body defenses.

Opportunistic Pathogens

Opportunistic pathogens are normal microbiota that cause disease under certain circumstances, such as:

• Immune suppression

• Changes in the normal microbiota

• Stressful conditions

Reservoirs of Infection

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

Portals of Entry and Exit

Pathogens enter and exit the body through specific sites:

• Skin: Outer layer acts as a barrier; some pathogens enter through cuts or by digesting outer layers.

• Placenta: Pathogens may cross, causing spontaneous abortion, birth defects, or premature birth.

• Mucous Membranes: Respiratory tract is the most common site; gastrointestinal tract is also common.

Adhesion in Disease

Adhesion is the process by which microorganisms attach to cells. It is essential for colonization and infection.

• Adhesion factors include specialized structures (adhesins, ligands) that bind to host cell receptors.

• Biofilms: Some bacteria attach to each other to form biofilms (e.g., dental plaque).

Etiology and Koch’s Postulates

Etiology is the study of the cause of disease. Koch’s postulates are criteria to establish a causative relationship between a microbe and a disease, but there are exceptions (e.g., multiple pathogens, unculturable microbes).

Virulence Factors

Virulence is the degree of pathogenicity. Virulence factors contribute to a microbe’s ability to cause disease:

• Adhesion factors: Enable attachment to host cells.

• Biofilms: Communities of microorganisms attached to surfaces.

• Extracellular enzymes: Help pathogens invade or evade host defenses.

• Toxins: Chemicals that harm tissues or trigger immune responses.

• Antiphagocytic factors: Prevent destruction by host phagocytes (e.g., capsules, chemicals).

Toxins: Exotoxins vs. Endotoxins

Toxins are classified into two groups:

• Exotoxins: Soluble proteins secreted by pathogens; include cytotoxins, neurotoxins, and enterotoxins.

• Endotoxins: Lipopolysaccharide (LPS) from the outer membrane of Gram-negative bacteria; released upon cell death.

Stages of Infectious Disease

Modes of Disease Transmission

• Contact Transmission: Direct (person-to-person), indirect (fomites), droplet (within 1 meter)

• Vehicle Transmission: Airborne, waterborne, foodborne, body fluids

• Vector Transmission: Biological (e.g., mosquitoes), mechanical (e.g., flies)

Types of Disease

• Acute: Develops rapidly, short duration

• Subacute: Between acute and chronic

• Chronic: Develops slowly, long duration

• Latent: Appears long after infection

Communicable vs. Noncommunicable Diseases

• Communicable: Spread from host to host (e.g., flu)

• Noncommunicable: Not spread between hosts (e.g., tetanus)

Patterns of Disease Occurrence

• Endemic: Constantly present in a population

• Sporadic: Occurs irregularly

• Epidemic: Sudden increase in cases

• Pandemic: Epidemic that spreads globally

Nosocomial (Healthcare-Associated) Infections

Nosocomial infections are acquired in healthcare settings during the course of receiving treatment.

Epidemiology: Key Terms

• Morbidity: State of being diseased or unhealthy

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

• Prevalence: Total number of cases (new and pre-existing) at a specific time

• Mortality: Number of deaths due to a specific disease

Additional info: These notes are based on standard microbiology textbook content and expand on the provided class notes for clarity and completeness.