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University of Ha'il Faculty of Public Health Foundation of Public Health Lecture No.: 8 INFECTIOUS DISEASE 1|Page INFECTIOUS DISEASE Definitions: Infectious diseases Communicable diseases Transmission – through unnatural routes – from an infected person Contamination Transmission – directly or indirectly – from an infected person Transmissible diseases Caused by an infectious agent The mere presence of microbes in or on the body. Infection Results when the organism has evaded the body’s external defenses, multiplied, and become established in the body Infection Is the invasion of the host by a pathogen. Disease Results only if the invading pathogen alters the normal functions of the body Disease is also referred to as morbidity Manifestations of Disease Symptoms 2|Page Subjective characteristics of disease felt only by the patient Objective manifestations of disease that can be observed or Signs measured by others Syndrome Group of symptoms and signs that characterize a disease or abnormal condition Asymptomatic, or subclinical, infections lack symptoms but may still have signs of infection Virulence Factors of Infectious Disease Pathogenicity Ability of a microorganism to cause disease. Virulence Degree of pathogenicity Virulence factors contribute to an organisms virulence Adhesion factors Biofilms Extracellular enzymes Toxins Antiphagocytic factors Susceptibility Ability to become infected with an organism when exposed to it People may loose susceptibility (or become immune) once they have been: infected with the organism vaccinated Medications may reduce susceptibility temporarily 3|Page The source and/or reservoir of the pathogen . Source: The location from which the pathogen is immediately transmitted to the host directly or indirectly Reservoir : The site or natural environmental location in which the pathogen resides and from which infection of the host can occur Epidemiologic Triad Disease is the result of forces within a dynamic system consisting of: AGENT OF INFECTION HOST ENVIRONMENT Factors Influencing Disease Transmission AGENT OF INFECTION Infectivity Pathogenicity Virulence Immunogenicity Antigenic stability Survival 4|Page HOST Age Sex Genotype Behaviour Nutritional status Health status ENVIRONMENT Weather Housing Geography Occupational setting Air quality Food 1. Infectious Agents Viruses Fungi Protoctists / Protozoa Helminths 2. 3 conditions required for infection to spread from one person to another: 1. One person must be infected with a microorganism. 2. The other person must be susceptible to infection with that microorganism. 5|Page 3. The microorganism must be able to leave the body of the infected person and enter the body of the susceptible person. 3. The infectious Disease cycle 4. Chain of Infection 5. Spread of infection Three conditions must be met for infection to spread from person to person If any of these conditions are not met, the chain is broken and the infectious disease does not spread. 6. Spectrum of Disease Exposure Subclinical manifestations Pathological changes Symptoms Clinical illness Time of diagnosis Death Whether a person passes through all these stages will depend upon infection and prevention, detection and therapeutic measures 7. Terms Used to Classify Infectious Diseases 8. Portals of Entry Sites through which pathogens enter the body Four major types Skin Mucous membranes Placenta Parenteral route Skin Outer layer of packed, dead, skin cells usually acts as a barrier to pathogens Some pathogens can enter through openings or cuts Others enter by burrowing into or digesting the outer layers of skin Mucous Membranes 6|Page Line the body cavities that are open to the environment Provides a moist, warm environment that is hospitable to pathogens Respiratory tract is the most commonly used site of entry – entry is through the nose, mouth or eyes Pathogens able to survive the acidic pH of the stomach may use the gastrointestinal tract as a route of entry Mode of transmission The way a microorganism is spread from an infected person to a susceptible person DIRECT TRANSMISSION INDIRECT TRANSMISSION Direct contact Airborne Droplet spread Vehicle borne Vector borne Direct transmission The spread of an organism from one person to another as a result of the direct contact of an infected and a susceptible person The modes of direct transmission direct contact droplet spread Direct contact The organism is passed to a susceptible person when he or she has contact with an infected person’s skin, mucus membranes or body fluid (blood, urine, feces). Droplet spread The organism is spread to a susceptible person in respiratory secretions that form when an infected person breaths, coughs or sneezes. 7|Page A susceptible person can become infected if the droplet enters his eyes, nose or mouth directly or via contamination of hands. Considered direct transmission because droplets are heavy so they do not tend to travel more than 6 feet from the infected person A susceptible person who is not near an infected person will not become infected The spread of disease by respiratory droplets usually requires extended contact Indirect transmission Occurs when an organism is spread from an infected person to a susceptible person via an intermediary or go-between. Via the air = airborne Via an insect = vectorborne Via an inanimate object = vehicleborne Mode of Transmission Person-to-person (respiratory, oro-genital, skin) Examples: HIV, measles Vector (animals, insects) Examples: rabies, yellow fever Common vehicle (food, water) Examples: salmonellosis Mechanical vectors (personal effects) such as doorknobs, or toothbrushes are called fomites The course of an infectious disease: 9. 1. The incubation period is the period between pathogen entry and the expression of signs and symptoms. The pathogen is spreading but has not reached a sufficient level to cause clinical manifestation. This period’s length varies with disease 8|Page 10. 2. the prodromal stage is the period in which there is an onset of signs and symptoms, but they are not yet specific enough to make a diagnosis. The patient often is contagious. 11. 3. The illness period is the phase in which the disease is most severe and has characteristic signs and symptoms. The immune response has been triggered; B and T cells are becoming active. 12. 4. in the periods of decline, the signs and symptoms begin to disappear. The recovery stage often is referred to as convalescence. CONTROL OF EPIDEMICS 1. There are three types of control measures. 1. The first type is directed toward reducing or eliminating the source or reservoir of infection: 9|Page Quarantine and isolation of cases and/or carriers Destruction of an animal reservoir of infection Treatment of sewage to reduce water contamination Therapy that reduces or eliminates infectivity of the individual 2. The second type of control measure is designed to break the connection between the source of the infection and susceptible individuals. Examples include : sanitation measures: Chlorination of water supplies Pasteurization of milk and other beverage Supervision and inspection of food and food handlers Destruction of Vectors 3. The third type of control measure reduces the number of Susceptible individuals and raises the general level of herd immunity by immunization. Example include: Passive immunization to give a temporary immunity following exposure to a pathogen or when a disease threatens to take an epidemic form Active immunization to protect the individual from the pathogen and the host population from the epidemic 10 | P a g e Levels of Disease Occurrence Sporadic level: occasional cases occurring at irregular intervals Endemic level: persistent occurrence with a low to moderate level Hyperendemic level: persistently high level of occurrence Epidemic or outbreak: occurrence clearly in excess of the expected level for a given time period Pandemic: epidemic spread over several countries or continents, affecting a large number of people 11 | P a g e