Epidemiology Epidemiology: Science of monitoring disease and understanding its causes (etiology) and transmission. Incidence rate of a disease: Total number of new cases seen within a calendar year Prevalence of a disease: Number of people infected at any given time Prevalence rate: Total number of new and old cases of a disease Morbidity rate: State of illness or the number of people in a given population that are ill Mortality rate: Number of people that are dead Epidemic: Number of new cases of a particular disease is greater than what is normally expected in the population Pandemic: When epidemics spread around the globe and affect more than one continent or population Endemic: Present to only specific areas in the World Sporadic: Present randomly Symptomatic: Feeling sick Clinical symptoms: Clearly evident symptoms Disease carriers: Individuals infected with a disease Subclinical carriers: Disease carriers who never develop clinical symptoms Incubatory carriers: Disease carriers who transmit the disease before developing symptoms Convalescent carriers: Disease carriers who are recovering from the disease, but can infect others Chronic carriers: Disease carriers who develop chronic infections and transmit the infection for long periods of time. Fomites: Contaminated inanimate objects Portals of entry: The sites where microorganisms enter the body Portals of exit: The sites where microorganisms leave the body Resistance: Ability to ward off disease Susceptibility: Chances of becoming infected Incubation period: Time between initial exposure and start of infection to the first appearance of signs and feelings of symptoms Prodromal period: Period of mild symptoms Period of Illness: Acute phase of the disease Signs: Objective findings that an observer or physician can see Period of decline: Individual’s signs and symptoms subside Period of convalescence: Phase where recovery has occurred
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Immunity A. Herd Immunity 1. As a pathogen infects members of a host population, the survivors acquire immunity to the pathogen. As more members of the population become immune, it is harder for the pathogen to spread from person to person 2. It provides protection for people without immunity because the people with immunity can’t spread the pathogen 3. Immunizing a few people won’t be sufficient to protect the population because too many people can serve as hosts for the pathogen.
4. Immunizing more people can effectively block pathogens from spreading through the population 5. Example scenario a. A group with mostly immunized people, a few current carriers of a pathogen and a few people without immunity. The people with the disease are relatively isolated because of the “healthy” people around them. So, the disease can’t spread because the immunized people effectively stop the disease before it infects them. B. Body 1. When a pathogen enters the body, its proteins act as antigens and stimulates the production of defensive proteins called antibodies. 2. The defensive antibody binds to the antigens preventing them from attacking any other cells and marking them for destruction. As the host gets well, he/she becomes immune. 3. Relating to herd immunity a. At the beginning of an epidemic, most people don’t have immunity, but as it progresses, more people develop immunity. When enough people become immune, that epidemic ends. C. Mutations 1. Antigenic drift is when small changes occur in the antigens 2. This occurs naturally, leading to multiple epidemics because old antigens no longer work. 3. Viruses a. During and in between seasons of epidemics, different viruses infect the same cells. The viruses can then exchange different RNA strands, creating new strands. The people with previous immunity are no longer immune because the previous antigens have no effect on the new pathogens. 4. Antigenic shift major changes in flu antigens D. Coevolution of Host and Pathogen 1. As pathogens and their host survive for a long time, they may coevolve. a. Highly virulent (dangerous) strains can result in rapid mortality and therefore not spread as widely, because its host doesn’t come in contact with enough people b. Less virulent (dangerous) strains can spread more widely because its hosts come in contact with more people. II.
Reservoirs A. Animals 1. Microorganisms can infect animals and humans 2. Many disease causing agents use animals to infect humans a. Apes and monkeys serve as reservoirs for human infection because they’re physiologically similar to humans
3. A disease transmitted from domestic and wild animals to humans is called zoonosis 4. Humans become infected with direct contact with animals, their wool, or hides, contaminated soil, inhalation of spores, and ingestion of meat or milk. 5. Humans and domestic animals can also be reservoirs for wild animals. B. Nonliving Reservoirs 1. Examples a. Water and soil III.
Disease Transmission A. Mechanics 1. A disease must have a portal of exit from the reservoir and a portal of entry to the infected host. That’s how diseases are spread, and how new cases of infection occur a. Respiratory, digestive, urinary, skin tract 2. Disease spread by three different modes of transmission: contact transmission, vehicle transmission, and vector transmission B. Contact Transmission 1. Contact transmission of a disease-causing agent can either be direct or indirect 2. Direct contact transmission occurs from skin to skin contact a. Shaking hands, intercourse, touching open sores 3. Indirect contact transmission occurs when infection is spread through any nonliving, inanimate object 4. These contaminated inanimate objects are called fomites 5. Droplet transmission occurs through sneezing, coughing, and speaking in close contact with an infected individual. a. Examples of diseases: pneumonia, influenza, common cold, and whooping cough C. Vehicle Transmission 1. In vehicle transmissions, pathogens can be spread through the air and in water, food products, and body fluids 2. Airborne a. Airborne microorganisms mainly come from animals, plants, water, and soil. These microorganisms can transmit disease through air b. Airborne pathogens have the greatest change of infecting new individuals when people are crowded together indoors, where heating and air conditioning units regulate temperature and very little fresh air enters the building i. Airborne pathogens can fall to the floor and combine with dust particles. 1. This dust can then be stirred up with walking, dry mopping, or changing bedding and clothing a. Measles, chickenpox, tuberculosis
3. Waterborne a. Waterborne microorganisms that cause diseases do not grow in pure water, they thrive in polluted water because polluted water is rich in nutrients b. Waterborne pathogens are usually transmitted in contaminated water supplies due to untreated or inadequately treated sewage i. Cholera c. Indirect fecal-oral transmission of pathogens occurs when the disease-causing microorganism living in the fecal matter of one organism infects another organism d. Waterborne diseases generally infect the digestive system, causing gastrointestinal signs and symptoms 4. Foodborne a. Foodborne pathogens are normally transmitted through improperly cooked or improperly refrigerated food, or unsanitary conditions i. Salmonellosis, typhoid fever, tapeworm b. Improper hygiene of food handlers also plays a key role in foodborne transmission c. Foodborne pathogens can produce gastrointestinal signs and symptoms D. Vector Transmission 1. Vector spread is the transmission of an infectious agent by a living organism to humans 2. Most vectors are ticks, flies and mosquitoes (arthropods) 3. Vectors can transmit disease in two ways (mechanical and biological) a. Mechanical vectors can passively transmit disease with their bodies i. These animals commonly feed on fecal matter ii. They then fly to feed on human food, transmitting pathogens along the ay iii. Keeping mechanical vectors away from food preparation and eating areas are means of prevention b. Biological vectors can actively transmit disease-causing pathogens that complete part of their life cycle within the vector i. In most vector-transmitted diseases, a biological vector is the host for a phase of the life cycle of the pathogen 1. Example of host organism: mosquito a. Yellow fever, typhus ii. To cause infection, a microorganism must enter the body (portal of entry) and have access to body tissues iii. The portal of entry is similar to the portal of exit for the host to be susceptible for a certain disease iv. Microorganisms can invade tissues directly or cross the placenta to infect the fetus
E. Connecting with Epidemiology 1. The transmission of disease by carriers causes epidemiological problems because carriers usually do not know they are infected and spread the disease, causing sudden outbreaks a. Carriers can transmit disease by direct and indirect contact or through vehicles such as water, air, and food. IV.
The Development of Disease A. Mechanics 1. In order for a pathogen to infect a host, there must be a susceptible host for the disease to be transmitted 2. If a host’s resistance is low, its susceptibility increases 3. Primary defense mechanisms of the body for resistance include intact skin (no cuts or abrasions), mucous membranes, a good cough reflex, normal gastric juices, and normal bacterial flora B. Incubation Period 1. If a microorganism penetrates these defenses the development of a disease process begins. 2. First there must be an incubation period a. Time between the initial exposure and start of infection to the first appearance of signs and feeling of symptoms 3. Different microorganisms have different incubation periods 4. Disease can be spread from the infected individual to a non-infected individual during this period C. Prodromal Period 1. After incubation period; mild symptoms D. Period of Illness 1. Acute phase of the disease 2. Individual presents with signs and symptoms of the disease 3. Signs are objective findings that an observer or physician can see a. Physical changes that can be measured 4. Symptoms are subjective and cannot be seen by an observer a. Changes in bodily functions b. White blood cells may increase, immune system responds to combat the disease-causing pathogen c. If the individual’s defense mechanism of the immune system does not successfully overcome the disease or if the disease is not treated properly, the person can die E. Period of Decline 1. Individual’s signs and symptoms subside and the person feels better a. Takes 24 hours to several days b. During this time, the individual is prone to secondary infections F. Period of Convalescence 1. Phase where recovery has occurred
2. The body regains strength and is returned to a state of normality. During this phase, infection can also be spread.