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are responsible for approximately ∼70% of invasive cervical cancers worldwide [ 4 ].
In contrast with the HR types, the LR HPV types, notably types 6 and 11, cause
almost all clinically visible benign lesions, that is, genital warts (flat condyloma and
condyloma acuminata) and laryngeal papillomas.
3.2 Epidemiology
In general, HPV is associated with more than 90% of anal and cervical cancers,
about 70% of vaginal and vulvar cancers, 70% of oropharyngeal cancers, more than
60% of penile cancers, and more than 10% of oral cavity cancers [ 5 ]. In addition,
specific HPV types also have been linked to certain cutaneous cancers [ 1 ]. On a
global scale, HPV accounts for approximately 600,000 cases of cervical cancer,
oropharyngeal cancers, anal cancers, vulvovaginal and penile cancers, as well as a
genital wart and recurrent papillomatosis of the lungs worldwide [ 5 ].
HPV could be transmitted through direct skin-to-skin, skin-to-mucosa, andmucosa-to-mucosa contact with an HPV-infected individual. The transmission is
correlated with sexual intercourse, both vaginal and anal. In addition, transmission
can also occur following non-penetrative sexual activity such as oral–genital sex
[ 6 ]. It is less common that HPV can occur via vertical transmission from mother to
infant during the intrapartum period [ 2 ], and occasionally transmission even through
fomites has been known to occur [ 7 ].
Risk factors for the development of HPV infection include sexual activity, mul-tiple sex partners, lack of condom use, use of oral contraceptives, age at first sexual
intercourse, non-monogamous sexual relationships, and history of prior infection
[ 8 ]. In addition, low socioeconomic status and smoking habit of individuals have
reported to increase the risk of acquiring the infection.
The estimated global prevalence rate of genital HPV infection is 12% [ 9 ].However, in most cases, the infection of HPV is subclinical, and 60–70% of new
infections are cleared within 1 year and 91% are cleared within 2 years [ 10 ]. Only a
small proportion of HPV infections progress to persistent infection, often involving
HR HPV types. Among sexually active individuals, the lifetime risk of HPV infec-
tion is ~75%. The HPV risk and prevalence vary by age and gender. Genital HPV
infection is very common in the younger age group (<25 years old). Sexually active
adolescents and young adult females are at a higher risk compared to women and
men greater than 25 years of age [ 10 , 11 ]. However, the more clinical cases were
studied in women than that in men, although the natural rate of the HPV infection is
similar in both populations.
Y. Li and C. Xu