There are more than 150 types of HPV. They usually don't cause any problems. However, when they do, the most frequent effect is the common wart, such as those found on the hands and feet.
About 30 HPV types are spread through genital contact. Each is “named” with a number, in the order of their discovery. In addition, they are divided into two groups:
"Low-risk" types of HPV
There are about 12 types of HPV that are called "low risk" because they are not known to cause cervical cancer. They can, however, cause genital warts or very minor cell changes on the cervix. These low-risk types of HPV are known by the numbers 6, 11, 40, 42, 43, 44, 53, 54, 61, 72, 73 and 81. Types 6 and 11 – which are linked to about 90 percent of genital warts – are the most common.
"High-risk" types of HPV
There are more than a dozen types of "high-risk" HPV that can cause abnormal cells to form on the cervix. These abnormal cell changes may gradually develop into cervical cancer if not removed. The 13 types of high-risk HPV that are of most concern are known by the numbers 16, 18, 31, 33, 35, 39, 45, 51, 52, 56, 58, 59 and 68. Types 16 and 18 are the most dangerous, since they cause about 70 percent of cervical cancers. In one study, the American National Cancer Institute found that about 10 percent of women with HPV type 16 or 18 developed advanced, pre-cancerous cervical disease (CIN 3) within three years (compared to just 4 percent of women with any type of HPV), and 20 percent did so in 10 years (compared to 7 percent).