Transmission of disease medium

Koch's Postulates

• To persist, pathogens must leave an infected host, survive transmission, enter a susceptible host, and develop/multiply within the new host.

Chain of Transmission

• Requires an agent (pathogen), reservoir (where it lives), exit route, transmission mode, entry portal, and susceptible host.

Classifying Transmission

• Horizontal: Person-to-person
• Vertical: Mother-to-child

Direct and Indirect Transmission

• Direct: Physical contact (kissing, touching)
• Indirect: Through objects or vectors (air, water, food, fomites, vectors). Pathogen survival time outside the host dictates transmission type.

Indirect Transmission: Vehicles

• Air/droplets (e.g., tuberculosis, measles): Airborne transmission for pathogens resistant to drying. Droplets quickly settle, unlike dust particles
• Water (e.g., typhoid): Contamination via human/animal excreta.
• Soil (e.g., tetanus): Spores survive for long periods, transmitting intestinal worms.
• Food (e.g., salmonella): Contamination often via food handlers with poor hygiene.
• Fomites (inanimate objects): Indirect contact transmission.

Indirect Transmission: Vectors

• Insects: Many diseases are caused by organisms transmitted through insect bites or feces and secretions. Mosquitos (malaria, West Nile virus, etc.) represent a significant vector. Soil is sometimes considered to act as a vector
• Accounts for 4.1% of global disease burden, causing 1.8 million deaths annually.

Host Defense Mechanisms: Preventing Ingress

• Mucociliary escalator: Mucus and cilia in the respiratory tract trap and remove microbes.
• Skin: Keratinized outer layer acts as a physical barrier, except for unkeratinized areas (alimentary and respiratory tracts, eyes, gingival sulcus).
• Fatty acids and salts (skin secretions): Kill bacteria.
• Saliva: Contains antibacterial agents and has a lavage effect.
• Gingival crevicular fluid: Antibacterial solution with immunological defense mechanisms.
• Blood: Flushing action and clotting mechanisms prevent infection spread.
• Blood-brain barrier: Specialized filter protecting the brain and spinal cord.

Host Defense Mechanisms: Dealing with Ingress

• Immune response: Recognizes and destroys foreign substances, distinguishing between self and non-self. Memory cells facilitate faster responses upon re-exposure to antigens. This varies based on lifestyle factors such as diet, smoking, and stress.

Immune System Cells: Lymphocytes

• White blood cells circulating in the lymphatic system.
• B lymphocytes: Produce antibodies (immunoglobulins) for humoral immunity. Plasma cells produce antibodies, memory cells provide long-term immunity.
• T lymphocytes: Control immune response and destroy antigens directly (cell-mediated immunity).

Host Response to Biofilms

• Inflammation (redness) indicates biofilm presence. Oral hygiene removes biofilm.