Human papillomavirus infection is characterised in cervical smears by the presence of koilocytes which are pathognominic for this infection. Koilocytes are mature  squamous cells with a characterisitic clear halo surrounding the nucleus.The halo is well defined and surrounded by a sharp edge of eosinophilic cytoplasm .The nuclei may be multiple and are slightly enlarged reflecting viral replication. According to the Bethesda System  smears containing koilocytes are classified as ASC-US or LSIL .