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Cervical cancer accounts for 10 per cent of all female cancers worldwide, and in developing countries it ranks as the most or second-most common form of cancer of women.

In Canada, however, it is the 12th most common cancer diagnosed in women of all ages, and the number of new cases and deaths has steadily declined since the 1970s.

In 2004, we saw about 1,350 new cases of cervical cancer in Canada, and 410 deaths. The rate of new cases and the mortality rates have been cut in half in the past 30 years, thanks mainly to the Pap test.

But the news isn't all good. For Canadian women under 50, cervical cancer is the second most common form of cancer -- a shocking statistic on par with developing countries.

The Pap smear test -- in which a sample of cells from the cervix is examined for abnormalities -- is the most important tool in early detection of cervical cancer, yet only about 80 per cent of women in Ontario have had a recent test. More than half of all cases of cervical cancer occur in women who have never had a Pap test.

The cervix is the narrow lower end of the uterus, leading into the vagina.

When your doctor uses a speculum to do an internal exam, the cervix can be clearly seen. The part of the cervix closest to the uterus is called the endo-cervix; the area closer to the vagina is the ecto-cervix.

The place where the two meet is called the "transformation zone," and it is from here that a swabbing of cells is taken for a Pap test.

We now know that the primary cause of cervical cancer is a sexually transmitted infection, caused by the human papilloma virus. HPV is present in both men and women, and is spread through sexual contact.

While there are more than 40 types of HPV known to infect the lining of the anogenital tract, 13 are considered to be high-risk, cancer-causing strains.

While it is clear that HPV is required to cause cervical cancer, other risk factors are involved. As Cancer Care Ontario notes, these include commencing sexual relations at an early age; having sexual partners who are HPV carriers; and having a large number of partners. Other factors that have been linked to cervical cancer include tobacco smoking (and exposure to second-hand smoke); use of oral contraceptives for more than five years; having other sexually transmitted infections such as chlamydia, herpes or HIV; eating a poor diet; having a suppressed immune system; and having more than five full-term pregnancies.

All women who are, or have ever been, sexually active should have Pap tests.

This screening should begin within three years of a woman's first vaginal sexual activity and should be done annually for three consecutive clear tests.

After that, Pap tests should be done every two to three years and may be discontinued after the age of 70 if there is an adequate negative screening history in the previous 10 years. If an abnormality is detected during a Pap test, you should be retested every six months for two years, Health Canada says.

(Women in special circumstances -- such as those with compromised immune systems, those who have a partial or total hysterectomy, those who are pregnant, or those who have same-sex relations -- should talk to their doctor about how often their tests should be done.)

Condoms can help prevent transmission of infections such as herpes.

A recent study also suggests that the use of condoms can dramatically reduce, but not completely eradicate, the chance of spreading HPV.

The study, published in the New England Journal of Medicine, found that women who said their male partners always used condoms were 70 per cent less likely to develop an HPV infection than women whose partners used them less than 5 per cent of the time.

The HPV can also cause genital warts, which are highly contagious. It's estimated that about two-thirds of people who have intimate contact with an infected partner will develop warts within three months.

Most HPV infections remain latent, meaning they are present but do not cause any tissue changes that can lead to cancer. While it isn't known how long HPV can persist in humans, it is known that there can be spontaneous regression and even clearance of an infection, or the lesions that develop on the cervix. Women under the age of 30 are more likely to show such regression, usually with low-grade lesions. However, about 2 per cent of sexually active women will develop a persistent HPV infection and thus be at a higher risk for developing cervical cancer.

It is now known which types of HPV are likely to cause cervical cancer, and American scientists recently unveiled a preventive vaccine that targets two types of HPV responsible for about 70 per cent of cervical cancers. The vaccine is licensed in the United States for use in females ages nine through 26, so they can develop antibodies to the virus before they are exposed to infection through sexual relations. (The vaccine also blocks infection by two other HPV strains responsible for 90 per cent of genital wart cases.)

The new vaccine may not protect women already infected and, in fact, may increase their risk of the kind of lesions that can lead to cervical cancer, the U.S. Food and Drug Administration warns.

While this vaccine is an exciting development in the move toward eradicating cervical cancer, regular Pap tests are still the essential tool for early detection and treatment.

Dr. Marla Shapiro can be seen Tuesdays on CTV's Canada AM. Questions about general health issues can be sent to her at: health@globeandmail.com (Please direct queries about personal health issues to your doctor.)