Distinctions are often made as to the type and duration of infectious organisms that infect the body of host organisms. Normal or Resident Flora - organisms that are constantly present in or on the body; normally they exist in homeostatic balance with the host and do not cause disease; however, they must be taken into consideration during operations that pierce or depress the body defenses and represent ever present sources of potential contamination. Transient Flora - organisms that reside in or on us for varying lengths of time; they may or may not cause disease dependent upon the resistance of the host and the virulence of the organism. The best known method for attempting to determine the causal agent of disease was set forth by Robert Koch. At the time he promulgated his postulates they represented a logical set of guidelines. As general rules of protocol they are still useful but we are now aware of many exceptions to Koch's Postulates. 1. The organism that is causing the disease should always be present in diseased animals and absent in healthy animals. Exception to Postulate 1 - organisms responsible for disease symptoms may be part of the resident flora and constantly present 2. The organism causing the disease must be isolated and grown in pure culture in the laboratory (on artificial media). Exception to Postulate 2 - certain organisms have never been cultured successfully on artificial laboratory media - syphilis, leprosy 3. A pure culture of the disease producing organism, when inocu- lated into healthy, susceptible animals, should cause the disease Exception to Postulate 3 - humans are not experimental animals and with the exception of laboratory accidents this postulate cannot be applied to humans; a reaction may not occur in the host if it has been previously sensitized and possesses protective antibodies 4. The pathogen must again be isolated from the inoculated animal and shown to be the original organism. Exception to Postulate 4 - the organism may have been cleared from the system as the result of antibody activity or the symptoms may be caused by toxins produced by an organism which is now dead or out of the body Koch's Postulates were designed with the idea that there was one and only one organism responsible for a given set of symptoms. This is the highest level of specificity and it was thought that if this were true then a single type of chemical agent would be found or needed to cure each given disease. As we shall see there are many levels of specificity. A. specific disease - specific organism -- one and only one organism causes a distinct set of unique symptoms, e.g. Treponema pallidum,Corynebacterium diphtheriae, Clostridium tetani B. general symptoms - several organisms -- similar symptoms are elicited from a variety of organisms; most inflammatory reac- tions fall into this category; e.g. nephritis, meningitis, endocarditis, peritonitis may be caused by Escherichia coli, Staphylococcus aureus, Streptococcus faecalis, Pseudomonas sp. C. several diseases - single organism -- one organism may be responsible for causing more than one particular disease; e.g. Mycobacterium tuberculosis can cause pulmonary tuberculosis or infections of the skin, bones, or internal organs To be pathogenic, a microorganism must in some way cause harm to the host. In most cases, a definite sequence of events occurs during infection and disease. 1. transfer of the parasite to the host (DISPERSAL) 2. entry of the parasite into the host's tissues and an increase in the number of parasites (INVASION) 3. injury to the host (PATHOGENICITY) 4. response of the host to the presence of the parasite (IMMUNITY) The spread of infectious microorganisms through a population depends upon a number of factors including the reservoir of infection of the microbes. These reservoirs may be classified as:human, animal, inanimate or non-living. a. Human reservoirs - all human beings carry microbes capable of disease transmission under certain conditions; some indivi- duals harbor pathogens and transmit them to others but do not show any symptoms of the disease organism themselves; these individuals, called CARRIERS, fall into one of three categories - you will recognize yourself in one or more of these situations. 1. healthy carriers - harbors infectious agents of disease but suffers no ill effects - best known example is Mary Mallon alias Typhoid Mary; many of us carry Group A streptococci in our nose and throat 2. incubatory carrier - may be infectious in later stages; only preliminary symptoms are present; e.g. sore throat, low grade fever - usually not enough to stop us from going to work and spreading the organisms; symptoms of German measles virus do not show up until after the infectious stage of the disease - we unwittingly spread it to others. 3. convalescent carrier - serve as a source of infection while recovering; even though we are not completely recovered we may go back to work out of misguided loyalty or fear of tyrannical employers or supervisors. b. Animal reservoirs - usually called ZOONOSES or ZOONOTIC infections; these are disease organisms which primarily affect lower animals but can be transmitted to humans by natural means such as fur, hides, feathers, consumption of infected animal products; domesticated animals are lively source of zoonoses c. Non-living or inanimate reservoirs are sometimes called FOMITES and can include pencils, pens, money, eating utensils, bottles, toys (dolls), postage stamps, door handles, linens, towels, clothing, toilet articles, papers, magazines, to name but a few. STAGES IN THE TRANSMISSION OF PATHOGENS Common Sources of Pathogens Human patients Human Carriers Animals | | Some Materials which may contain Pathogens Skin, Scales, Scabs, Hair, Animal Flesh, Sputum Droplets, Discharge from wounds, lesions, blood, urethra, vagina, feces, urine, eggs, milk | | Common Means of Transfer Direct contact, Fomites, Air, Dust Insects, Food, Water, Soil | | Common Portals of Entry into New Hosts Skin - usually via abrasions, wounds, burns, bites, glands and hair follicles Respiratory Tract - following inhalation Alimentary Canal - following ingestion Genital Tract - following sexual intercourse Transmission is often characterized as either: A. Mechanical - insects or other vectors physically transport a pathogen from contaminated materials such as food or water to other objects (cockroaches, flies) food, fingers, flies, feces, fomites, fornication, fluids, felines, feet, fleas, feathered friends, fingernails, fish, folks B. Biological - a portion of the pathogen's life cycle is carried out in the vector (_ anopheline mosquito and the malarial parasite); transmission is affected by injection of blood or blood products, warm-blooded animal bites, arthropod bites, introduction of arthropod feces into bites or wounds It is conventional to divide infectious diseases into those that are ENDEMIC, i.e. always present to some extent in the community and EPIDEMIC, i.e. showing a sharp increase or concentration of cases in time and space. The term PANDEMIC is often used to describe a world-wide or widely distributed epidemic. In a well adjusted host-parasite relationship, subclinical infection is the rule, disease the exception, and death a rarity. An epidemic in the popular sense of the word is merely the prevalence of a particular type of infection which appears to be unusually concentrated in a defined area within a prescribed time period. The rate of spread of an epidemic at any moment is not only a function of the number or density of susceptible persons avail-able, but of the number of sources of the infection as well. Epidemiology - is the study of the distributions and determinants of diseases prevalent in humans. Morbidity - number of individuals having the disease per unit of population (usually 100,000) within a given time period. Mortality - the number of deaths attributable to a particular disease per unit of population (usually 1,000) within a given time period. CONTROL OF INFECTIOUS DISEASE Certain characteristics of an infectious disease virtually rule out the possibility of world-wide eradication. 1. Infections of wild animals and birds where human infection results from intrusion into an alien ecosystem, e.g. jungle yellow fever, scrub typhus, psittacosis. 2. Infections that persist throughout the life of the carrier and are transmissible to others years after they were originally contracted. The two common human examples are Herpes simplex and Varicella-Herpes zoster. 3. Infections spread by the respiratory route in which the disease in question may be produced by viruses of many antigenic types. Certainly for influenza and probably in regards to the common cold viruses, such agents also have a high capacity to change antigenic character by mutation. This phenomenum of antigenic drift is particularly important in influenza. 4. Infections in which it is socially impossible to obtain public cooperation because they are seen as social and not medical problems. Venereal or sexually transmitted diseases (STD's) are the most common examples.