There are about 40 types of Human Papilloma Virus (HPV), the most common sexually transmitted disease.

. Worldwide, an estimated 630 million people are infected, including 20 million Americans. Between 28 million and 40 million sufferers have pre-cancerous infections.

. Four types of HPV are believed to cause most disease: HPV 16 and 18 cause about 70 per cent of cervical cancers, and HPV six and 11 cause 90 percent of genital warts.

. While genital warts are often visible, most people infected with HPV don't know because there are no symptoms. Regular Pap smears detect precancerous lesions and early cervical cancer, improving survival odds.

. Two drug companies are working on vaccines. Merck & Co's Gardasil, which could get US approval in early June, blocks infection by HPV 6, 11, 16 and 18.

GlaxoSmithKline is testing Cervarix, which blocks HPV 16 and 18, and expects to apply for US approval by the end of the year.

. Even when the vaccines are ready, routine Pap screenings will still be needed because other types of HPV cause about 30 per cent of cervical cancers.

. Men infected with HPV can get cancer of the penis, the anus, the mouth and throat.

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