▁rate , ▁resulting ▁in ▁what ▁is ▁known ▁as ▁a ▁" vir gin ▁soil " ▁ep ide mic . ▁Before ▁the ▁European ▁arrival , ▁the ▁Amer icas ▁had ▁been ▁isolated ▁from ▁the ▁E uras ian - A fr ican ▁land mass . ▁The ▁pe op les ▁of ▁the ▁Old ▁World ▁had ▁had ▁thousands ▁of ▁years ▁for ▁their ▁populations ▁to ▁accommod ate ▁to ▁their ▁common ▁dise ases . ▁ ▁The ▁fact ▁that ▁all ▁members ▁of ▁an ▁imm un olog ically ▁na ive ▁population ▁are ▁exposed ▁to ▁a ▁new ▁disease ▁simultaneously ▁increases ▁the ▁fatal ities . ▁In ▁populations ▁where ▁the ▁disease ▁is ▁en demic , ▁gener ations ▁of ▁individuals ▁acquired ▁imm unity ; ▁most ▁adult s ▁had ▁expos ure ▁to ▁the ▁disease ▁at ▁a ▁young ▁age . ▁Because ▁they ▁were ▁resist ant ▁to ▁rein fection , ▁they ▁are ▁able ▁to ▁care ▁for ▁individuals ▁who ▁caught ▁the ▁disease ▁for ▁the ▁first ▁time , ▁including ▁the ▁next ▁generation ▁of ▁children . ▁With ▁proper ▁care , ▁many ▁of ▁these ▁" child hood ▁dise ases " ▁are ▁often ▁surv iv able . ▁In ▁a ▁na ive ▁population , ▁all ▁age ▁groups ▁are ▁affected ▁at ▁once , ▁leaving ▁few ▁or ▁no ▁health y ▁care g ivers ▁to ▁nur se ▁the ▁sick . ▁With ▁no ▁resist ant ▁individuals ▁health y ▁enough ▁to ▁tend ▁to ▁the ▁ill , ▁a ▁disease ▁may ▁have ▁higher ▁fatal ities . ▁ ▁The ▁n atives ▁of ▁the ▁Amer icas ▁were ▁faced ▁with ▁several ▁new ▁dise ases ▁at ▁once ▁creating ▁a ▁situation ▁where ▁some ▁who ▁successfully ▁resist ed ▁one