ized ▁hyp ers ens itivity ▁re actions ▁to ▁an ▁aller gen . ▁At opy ▁appears ▁to ▁show ▁a ▁strong ▁her edit ary ▁component . ▁One ▁study ▁con cludes ▁that ▁the ▁risk ▁of ▁developing ▁at op ic ▁der mat itis ▁( 3 %) ▁or ▁at opy ▁in ▁general ▁( 7 %) ▁" in cre ases ▁by ▁a ▁factor ▁of ▁two ▁with ▁each ▁first - deg ree ▁family ▁member ▁already ▁suffering ▁from ▁at opy ". ▁As ▁well , ▁mat ernal ▁stress ▁and ▁per in atal ▁programming ▁is ▁increasing ly ▁understood ▁as ▁a ▁root ▁cause ▁of ▁at opy , ▁finding ▁that ▁"... tra uma ▁may ▁be ▁a ▁particularly ▁robust ▁pot enti ator ▁of ▁the ▁cas cade ▁of ▁bi ological ▁events ▁that ▁increase ▁vulner ability ▁to ▁at opy ▁and ▁may ▁help ▁explain ▁the ▁increased ▁risk ▁found ▁in ▁low - in come ▁urban ▁populations .” ▁▁ ▁Environment al ▁factors ▁are ▁also ▁thought ▁to ▁play ▁a ▁role ▁in ▁the ▁development ▁of ▁at opy , ▁and ▁the ▁' h yg iene ▁hypothesis ' ▁is ▁one ▁of ▁the ▁models ▁that ▁may ▁explain ▁the ▁ste ep ▁rise ▁in ▁the ▁inc idence ▁of ▁at op ic ▁dise ases , ▁though ▁this ▁hypothesis ▁is ▁incomplete ▁and ▁in ▁some ▁cases , ▁contradict ory ▁to ▁find ings . ▁This ▁hypothesis ▁propos es ▁that ▁excess ▁' clean lin ess ' ▁in ▁an ▁infant ' s ▁or ▁child ' s ▁environment ▁can ▁lead ▁to ▁a ▁decl ine ▁in ▁the ▁number ▁of ▁inf ect ious ▁stim uli ▁that ▁are ▁necessary ▁for ▁the ▁proper ▁development ▁of ▁the ▁imm une ▁system