Hans Selye's experiments with rats shed pioneering light on how stress affects health. Toronto Public Library / Getty Images. By Lily Rothman.HistoryhealthMeettheDoctorWhoChangedOurUnderstandingofStressMeettheDoctorWhoChangedOurUnderstandingofStressHansSelye'sexperimentswithratsshedpioneeringlightonhowstressaffectshealthTorontoPublicLibrary/GettyImagesByLilyRothmanMarch10,201610:15AMESTThereleaseonThursdayoftheAmericanPsychologicalAssociation’sannualStressinAmericareportrepresentsnearlyadecadeoftheorganizationtrackingtheextentandimpactofstressinAmericans’lives.Thereportisclearevidencethatmedicinetakesstressseriously—aswellitshould—but,thoughhumanbeingshavealwaysfeltstress,it’sactuallybeenlessthanacenturysincethesubjectgottheattentionitdeserved.AsTIMEexplainedina1983coverstory,itusedtobethoughtthat“stress”wasjustavaguefeeling,notaprecisemedicalterm.Therewasnofirmdefinitionorwayofmeasuringit.Evenso,itwasclearthattherewassomethinggoingon.AsearlyastheCivilWar,aconditionknownas“soldier’sheart”wasnoticedbydoctors.“DuringWorldWarI,thecripplinganxietycalledshellshockwasatfirstattributedtothevibrationsfromheavyartillery,whichwasbelievedtodamagebloodvesselsinthebrain,”asTIMEputit.“ThistheorywasabandonedbythetimeWorldWarIIcamealong,andtheproblemwasrenamedbattlefatigue.”Whatearlierdoctorsstudyingsoldiershadmissedwasthatthelong-termactivationofthefamedfight-or-flightresponsecouldcauseproblemsthatenduredevenintimesofpeace.ThatchangedthankstoHansSelye,“thefatherofstressresearch.”SelyewasamedicalresearcherinMontrealwhostudiedhormonalchangesinratswhen,inthelate1930s,herealizedthattheratshewasstudyingwererespondingnotmerelytohisinjectionsofhormonesandplacebos,butalsotothestresscausedbytheexperiments.Itwasthatstressthatcausedtheratstobecomeillanddie.“His1936paperonstress,asthecauseofdeathinhisexperimentalrats,attractednomoreattentionthanAlexanderFleming’sfirstreportofpenicillin—anditmayprovenolessimportanttosufferingmankind,”TIMEwouldlaternote.Selyetheorizedthatoverexposingthebodytost