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Principles of Infectious Disease and EpidemiologyChapter 14

Outbreak, Warner Bros. Pictures (1995)1ObjectivesDefine an infectious diseaseDescribe the microbiome and its role in human healthUnderstand the classification of infectious diseasesList the modes of transmission of infectious diseases

2ObjectivesGive examples of factors which contribute to the development of emerging infectious diseasesList the methods by which outbreaks are recognizedDescribe the steps of conducting an outbreak investigationLearn the terminology used to describe an outbreak 3Triad of Infectious Diseases4The Human MicrobiomeHuman Microbiome ProjectThe human body contains 1013 human cells but harbors 1014 bacterial cellsNormal flora or normal microbiota are all the microorganisms which colonize the human bodyHMP-Started in 2007

5SymbiosisRelationship between two organisms in which at least one is dependent upon the otherCommensalismOne organism derives benefit while other is unaffectedMutualismBoth organisms benefit from symbiotic relationshipParasitism (tape worm)One organism benefits at expense of the otherOpportunistic pathogens (candida)Normal microbiota can become pathogenic in the right conditions6Symbiosis

What is the cause of this disease?

?8Traditional MicrobiologyTestTake all the suspected and isolateThe actual cause9Kochs PostulatesEstablishing a causal relationship between pathogen and infectious diseaseSame pathogen present in every case of the diseasePathogen must be isolated from diseased host and grown in pure culturePathogen from pure culture must cause disease in a healthy laboratory animalPathogen must be isolated from inoculated animal and shown to be the original pathogenFigure 14.3

10Exceptions to Kochs PostulatesSome diseases can be caused by multiple pathogens (pneumonia)Dysbiotic diseases - Perturbation of normal flora (yeast infection)Some agents can cause several disease conditions Streptococcus pyogenes (scarlet fever)Some agents are not culturable in the laboratory (leprosy)Some pathogens only cause disease in humans (small pox)

11Classifying Infectious DiseasesIncubation, Duration and SeverityAcute infection

Acute infectionlate complication(encephalitis)Latent infection

Chronic infectionviral sheddingChronic infectionlate diseaseSlow infection (madcow)Rhinovirus & influenzaMeaslesSSPEVaricellaZosterHerpes Simplex VirusHepatitis B VirusCancerCreutzfeldt-Jacob Disease (prion)time12Classifying Infectious DiseasesExtent of Host InvolvementLocal infectionPathogen limited to small area of bodySystemic infectionGeneralized infection affecting multiple organs spread via blood and/or lymphFocal infectionAgent travels through the blood or lymph but causes disease only in one site or organSubclinical diseaseNo noticeable signs or symptoms (potential carriers of the disease)13Classifying Infectious DiseasesExtent of Host InvolvementBacteremia - bacteria in the bloodViremia viruses in the bloodToxemia toxins in the bloodSepticemiaMultiplication of pathogens in the blood leading to sepsisSepsisGeneralized inflammatory condition affecting multiple organ systems14The Stages of a DiseaseIncubation before the appearance of any signs (objective changes) or symptoms (subjective)Prodrome early mild symptoms of diseaseIllness disease is most severeDecline signs and symptoms subsideConvalescence patient regains strength and the body return to its prediseased stateFigure 14.5

15Reservoirs of InfectionReservoirs allow for survival, replication and transmission of a pathogenHuman most human infectious diseases depend upon human reservoirsCarriers can be healthy (subclinical), in the incubation period, have active disease, or be in the convalescent phaseAnimal diseases transmitted from animals to human are called zoonosesNon-living soil, water, food, etc.16Transmission of DiseaseContact transmissionDirect contact (person-to-person spread)Horizontal physical contactVertical from mother to fetus/newborn (AIDS, yeast infection, gonorrhea)Indirect contactFomites inanimate objects that facilitate spread of a pathogenDroplet transmissionShort range droplet nuclei (