**4. Risk and protective factors**

In the social sphere, people who have yet to initiate sex life have a low probability of HPV infection. On the other hand, people who started their sex life earlier or have a promiscuous sexual life are more likely to have sexually transmitted infections, including HPV. In the economical sphere, it has been found that low levels of HPV infection were strongly related with higher income levels [4].

Early age initial sexual activity, oral sex, more than one sexual partner, not wearing condoms, infrequent use of condoms all pose important risks to any sexually transmitted infection, such as HPV. Homosexuals tend to have greater number of sexual partners than heterosexuals, and as such, they should be more commonly infected than heterosexuals, but studies have yet to confirm this trend [4, 8].

Smoking may pose a lower risk for infection with high risk oral and oropharyngeal HPV than it was previously believed probably because the oral mucosa keratinization makes it stronger against minor traumas and consequently HPV infection. On the other hand, high levels of alcohol consumption have been linked to a higher risk of HPV positivity, but this association needs further research in order to be more fully established [68].

Normally, people acquire HPV during their adolescent years, or when they approach their twenties, when they start their sexual life. An immune competent person can suppress HPV or even eliminate it, and suppressed HPV may be kept subclinical for years [8].

Among the leading protective factors, saliva stands out. It is composed of lysozymes, lacto‐ ferrin, IgA and cytokines which seems to be the reason behind low HPV transmission through self-inoculation, oral sex, and the virtually inexistence of transmission through kissing. It has been suggested that regular consumption of carotenoids would contribute to make HPV persistence less likely, and a high consumption of folic acid would reduce the risk of HPV infection [4].
