o cide ▁working ▁in ▁tan dem . ▁ ▁Vir ul ence ▁and ▁mort ality ▁ ▁Vir al ▁and ▁b acter ial ▁dise ases ▁that ▁kill ▁vict ims ▁before ▁the ▁ill ness es ▁spread ▁to ▁others ▁tend ▁to ▁f lare ▁up ▁and ▁then ▁die ▁out . ▁A ▁more ▁res il ient ▁disease ▁would ▁establish ▁an ▁equilibrium ; ▁if ▁its ▁vict ims ▁lived ▁beyond ▁in fection , ▁the ▁disease ▁would ▁spread ▁further . ▁A ▁similar ▁evolution ary ▁pressure ▁acts ▁upon ▁victim ▁populations , ▁as ▁those ▁lack ing ▁gen etic ▁resistance ▁to ▁common ▁dise ases ▁die ▁and ▁do ▁not ▁leave ▁descend ants , ▁whereas ▁those ▁who ▁are ▁resist ant ▁pro create ▁and ▁pass ▁resist ant ▁gen es ▁to ▁their ▁off spring . ▁For ▁example , ▁in ▁the ▁first ▁fifty ▁years ▁of ▁the ▁six teenth ▁century , ▁an ▁unus ually ▁strong ▁stra in ▁of ▁sy ph ilis ▁killed ▁a ▁high ▁proportion ▁of ▁inf ected ▁Europe ans ▁within ▁a ▁few ▁months ; ▁over ▁time , ▁however , ▁the ▁disease ▁has ▁become ▁much ▁less ▁vir ul ent . ▁ ▁Thus ▁both ▁inf ect ious ▁dise ases ▁and ▁populations ▁tend ▁to ▁evol ve ▁towards ▁an ▁equilibrium ▁in ▁which ▁the ▁common ▁dise ases ▁are ▁non - sym pt om atic , ▁m ild ▁or ▁manage ably ▁chron ic . ▁When ▁a ▁population ▁that ▁has ▁been ▁relatively ▁isolated ▁is ▁exposed ▁to ▁new ▁dise ases , ▁it ▁has ▁no ▁resistance ▁to ▁the ▁new ▁dise ases ▁( the ▁population ▁is ▁" bi olog ically ▁na ive "). ▁These ▁people ▁die ▁at ▁a ▁much ▁higher