Medical Microbiology

26 1 GeneralAspectsofMedicalMicrobiology

orinendemicforms,dependingonthetimeandplaceoftheiroccurrence.

Thefrequencyoftheiroccurrence(morbidity)isdescribedastheirincidence

andprevalence.Thetermmortalityisusedtodescribehowmanydeathsare

causedbyagivendiseaseinagivenpopulation.Lethalityisameasureofhow

life-threateninganinfectionis.Themostimportantsourcesofinfectionare

infectedpersonsandcarriers.Pathogensaretransmittedfromthesesources

tosusceptiblepersonseitherdirectly(person-to-person)orindirectlyviain-

ertobjectsorbiologicalvectors.Controlofinfectiousdiseaseswithinapop-

ulacemustbesupportedbyeffectivelegislationthatregulatesmandatory

reportingwhererequired.Furthermeasuresmustbeimplementedtopre-

ventexposure,forexampleisolation,quarantine,disinfection,sterilization,

useofinsecticides,anddispositionalprophylaxis(activeandpassiveimmu-

nization,chemoprophylaxis). &

EpidemiologicalTerminology

Epidemiologyinvestigatesthedistributionofdiseases,theirphysiological

variablesandsocialconsequencesinhumanpopulations,andthefactors

thatinfluencediseasedistribution(WorldHealthOrganization[WHO]defi-

nition).Thefieldcoveredbythisdisciplinecanthusbedefinedasmedical

problemsinvolvinglargecollectives.Theruleofthumboninfectiousdiseases

isthattheircharacteristicspreaddependsonthevirulenceofthepathogen

involved,thesusceptibilityofthethreatenedhostspeciespopulation,and

environmentalfactors.Table 1. 8 providesbriefdefinitionsofthemostimpor-

tantepidemiologicalterms.

Transmission,SourcesofInfection

Transmission

Pathogenscanbetransmittedfromasourceofinfectionbydirectcontactor

indirectly.Table 1. 9 liststhedifferentdirectandindirecttransmissionpath-

waysofpathogenicmicroorganisms.

Person-to-persontransmissionconstitutesahomologouschainofinfec-

tion.Theinfectionsinvolvedarecalledanthroponoses.Incasesinwhichthe

pathogenistransmittedtohumansfromothervertebrates(andoccasionally

theotherwayaround)wehaveaheterologouschainofinfectionandthe

infectionsareknownaszoonoses(WHOdefinition)(Table 1. 10 ).

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Kayser, Medical Microbiology © 2005 Thieme