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, while the other is unaffected. Example: Mites in human hair follicles.

Normal Microbiota: Resident vs. Transient

Normal microbiota (normal flora) are organisms that colonize the body’s surfaces without normally causing disease. They are divided into:

• Resident microbiota: Always present, mostly commensal, and 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:

• Nonliving reservoirs: Soil, water, and food.

• Human carriers: Asymptomatic infected individuals can be infectious to others.

• Animal reservoirs: Zoonoses are diseases naturally spread from animal hosts to humans.

Portals of Entry and Exit

Pathogens enter and exit the body through specific sites called portals. Major portals include:

• Skin: Outer dead skin acts as a barrier; some pathogens enter through openings or cuts.

• Placenta: Pathogens may cross the placenta and infect the fetus.

• Mucous membranes: Line body cavities open to the environment (e.g., respiratory, gastrointestinal tracts).

Portals of exit are often the same as portals of entry and are important for the spread of disease.

Adhesion and Disease

Adhesion is the process by which microorganisms attach themselves to cells, a critical step in causing disease.

• Adhesion factors include specialized structures and attachment proteins (ligands).

• 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 cause the same disease, some pathogens cannot be cultured).

Virulence Factors

Virulence factors contribute to a pathogen’s ability to cause disease. Types include:

• 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. Types: Exotoxins (secreted proteins), Endotoxins (LPS from Gram-negative bacteria).

• Antiphagocytic factors: Prevent destruction by host phagocytes (e.g., bacterial capsule).

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

Infectious diseases progress through several stages:

• Incubation period: Time between exposure and onset of symptoms.

• Prodromal period: Mild, nonspecific symptoms.

• Period of illness: Acute phase with most severe symptoms.

• Decline: Symptoms subside as the body fights infection.

• Convalescence: Recovery and return to health.

Modes of Disease Transmission

Transmission is the movement of pathogens from a reservoir to a host. Main modes include:

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

• Vehicle transmission: Airborne, waterborne, foodborne, or via body fluids.

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

Vector Types: Biological vs. Mechanical

• Biological vectors: Pathogen develops or multiplies within the vector (e.g., mosquitoes for malaria).

• Mechanical vectors: Pathogen is passively carried on the body of the vector (e.g., flies carrying bacteria on their legs).

Types of Diseases

• Acute: Develops rapidly, short duration.

• Subacute: Between acute and chronic.

• Chronic: Develops slowly, lasts a long time.

• Latent: Pathogen remains inactive for a time before causing symptoms.

Communicable vs. Noncommunicable Diseases

• Communicable: Can be transmitted from person to person (e.g., flu).

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

Disease Occurrence: Endemic, Sporadic, Epidemic, Pandemic

• Endemic: Constantly present in a population.

• Sporadic: Occurs irregularly and infrequently.

• Epidemic: Sudden increase in cases in a population.

• Pandemic: Epidemic that spreads across countries or continents.

Nosocomial (Healthcare-Associated) Infections

Nosocomial or healthcare-associated infections (HAIs) are acquired during medical care.

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.

Summary Table: Types of Transmission

Additional info: These notes are based on the provided images and have been expanded with standard academic context for clarity and completeness.