Acriticalfactorthatplaysapartinsusceptibilitytocoldsisage.Astudy,donebytheUniversityofMichiganSchoolofPublicHealth,revealedparticularsthatseemtoholdtrueforthegeneralpopulation.Infantsarethemostcold-riddengroup,averagingmorethansixcoldsintheirfirstyears.Boyshavemorecoldsthangirlsuptoagethree.Aftertheageofthree,girlsaremoresusceptiblethanboys,andteenagegirlsaveragethreecoldsayeartoboys’two.Thegeneralincidenceofcoldscontinuestodeclineintomaturity.Elderlypeoplewhoareingoodhealthhaveasfewasoneortwocoldsannually.Oneexceptionisfoundamongpeopleintheirtwenties,especiallywomen,whoshowariseincoldinfections,becausepeopleinthisagegrouparemostlikelytohaveyoungchildren.Adultswhodelayhavingchildrenuntiltheirthirtiesandfortiesexperiencethesamesuddenincreaseincoldinfections.Thestudyalsofoundthateconomicsplaysanimportantrole.Asincomeincreases,thefrequencyatwhichcoldsarereportedinthefamilydecreases.Familieswiththelowestincomesufferaboutathirdmorecoldsthanfamiliesattheupperend.Lowerincomegenerallyforcespeopletoliveinmorecrampedquartersthanthosetypicallyoccupiedbywealthierpeople,andcrowdingincreasestheopportunitiesforthecoldvirustotravelfrompersontoperson.Lowincomemayalsoadverselyinfluencediet.Thedegreetowhichpoornutritionaffectssusceptibilitytocoldsisnotyetclearlyestablished,butaninadequatedietissuspectedofloweringresistancegenerally.