Light micrograph of a cervical smear revealing infection with the human papilloma virus (HPV), the causative agent of genital warts that is implicated in the development of cervical cancer. In HPV infection (koilocytosis), large vacuoles develop around the nuclei of intermediate squamous cells. The vacuoles appear clear, with the blue- staining cytoplasm displaced to form a concave shape at the edge of the cells. An additional, non-specific feature of HPV infection is parakeratosis - the formation of cells with keratinized cytoplasms (dense red stain) and small dark nuclei.