Hong KongTwinsDiseases in TwinsTwins, MonozygoticTwins, ConjoinedTwins, DizygoticTwin Studies as TopicPregnancy, TwinChinaInfluenza, HumanInfluenza A virusInfluenza A Virus, H9N2 SubtypeSevere Acute Respiratory SyndromeInfluenza A Virus, H5N1 SubtypeHemagglutination Inhibition TestsOrthomyxoviridaeAsian Continental Ancestry GroupPregnancy, MultipleDisease OutbreaksRetrospective Studies