Principles of Disease and Epidemiology

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Chapter 14 – Principles of Disease and Epidemiology

Pathology Infection and Disease
1. Pathology is the scientific study of disease – concerned with cause (etiology) and manner in which disease develops (pathogenesis) - Also concerned with structural and functional changes brought about by disease and final effects on the body 2. Infection – invasion or colonization of the body by pathogenic microorganisms 3. Disease – when infection results in change in state of health Normal Microbiota –

1. Free of microbes in utero, at birth gain lactobacilli in intestine from mothers vaginal canal, E. coli taken through food and inhabits intestine thru life 2. Ten times more bacterial cells than human cells in the body 3. Normal flora – normal microbiota

4. Transient microbiota – present for a while but disappear 5. Distribution of normal flora dependent on physical and chemical factors, defenses of the host and mechanical factors Relationship between Normal Flora and Host

1. Microbial antagonism – aka competitive exclusion – normal microbiota ability to protect the host from overgrowth of harmful microorganisms 2. Normal microbiota and host live in symbiosis – one is dependent on the other 3. The three types of symbiosis – commensalism (one organism benefits, other unaffected), mutualism (both benefit) and parasitism (one harmed and one benefits) 4. Probiotics – live microbial cultures applied to or ingested that are intended to exert a beneficial effect Opportunistic Microorganisms

1. E. coli – normally harmless but in other parts of the body – cause UTI, pulmonary infections, meningitis or abscesses. 2. AIDS – compromises immune system – increases susceptibility to opportunistic infection e.g. Pneumocystis pneumonia

The Etiology of Infectious Disease –
Koch’s Postulates –
1. Koch established that microorganisms cause specific disease 2. Isolated microorganisms from diseased animal, grew in pure culture and identified, injected healthy animal, disease reproduced and identical 3. Experimental requirements – same pathogen must be present in every case of disease, pathogen must be isolated from diseased host and grown in pure culture, pathogen must cause disease when inoculated into healthy animal, pathogen must be isolated from inoculated animal and shown to be original organism

Exceptions to Koch’s postulates –
1. Unique culture environments – modified to establish etiologies of those that cannot be grown on artificial media e.g. viruses, syphilis, leprosy 2. Some disease such as tetanus have unequivocal signs and symptoms 3. Some disease may be caused by a number of microbes e.g. pneumonia and nephritis 4. Some pathogens cause several diseases e.g. S. pyogenes 5. Certain pathogens only cause disease in humans e.g. HIV Classifying Infectious diseases –

1. Every disease alters body structures and functions in particular ways ad are indicated by several kinds of evidences 2. Symptoms – changes in body functions
3. Signs – objective changes the physician can observe or measure e.g. lesions, swelling, fever and paralysis 4. Syndrome – a specific group of signs or symptoms that accompany a disease 5. Communicable disease – any disease that spreads from one host to another e.g. herpes, chickenpox, measles, typhoid fever and TB 6. Contagious diseases – Spread easily from host to host e.g. chickenpox 7. Noncommunicable Disease – caused by microorganisms that inhibit body and only occasionally produce disease or live outside the body and produce disease when introduced e.g. tetanus Occurrence of Disease –

1. Incidence – the number of people in a population who develop a disease during a particular period of time 2. Prevalence – number of people in a population who develop a disease at a specific time, regardless of when it first appeared – old and new 3. Sporadic disease – occurs only occasionally e.g. typhoid fever 4....
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