Human papillomavirus (HPV) is a common oncogenic virus that likes to infect squamous cells. In the skin, this leads to warts. In the GI tract, this leads to mouth and anal cancers. And in the cervix and vagina, this leads to cervical cancer and vaginal cancer. HPV is the most common sexually transmitted infection. There is no cure. 

HPV causes a steady low level of inflammation, like a slow burn. This reaction is so subtle that it can’t even be seen with the naked eye. Inflammation causes cells to divide and regenerate more frequently than usual. More division means more opportunities for mutations. That’s part of the reason why chronic untreated HPV infections lead to cancer. The other reason has to do with two of HPVs virulence factors, E6 and E7. They both contribute to the development of cancer. They do so by speeding up the cell cycle. E6 disables the tumor suppressor p53, and E7 separates Rb from the pro-growth molecule EF2. When the immune system is disabled, for example with AIDS, the oncogenic pace of HPV dramatically accelerates. Smoking is also a strong risk factor, perhaps because it is slightly immunosuppressive. 

There are dozens of different HPV strains. 

The 6 and 11 strains cause warts. Warts are fleshy blobs with a rough cauliflower-like, or verrucous, surface. Genital warts are sometimes called condyloma acuminata. Histologically, warts feature thickening of all the epidermal layers and the appearance of koilocytes (raisin nuclei).

The 16, 18, 31 and 33 strands are strongly associated with cervical dysplasia, which leads to cervical cancer. HPV likes to infect the border between the endocervix (columnar cells) and ectocervix (squamous), which is imaginatively called the squamocolumnar junction or transformation zone. The transformation zone moves up and down the cervix during a woman’s lifetime, dictated by the ebb and flow of hormones. 

Cervical cancer takes decades to develop, but the evidence is only visible histologically. The Pap smear, which involves scraping the surface of the cervix and inspecting the scrapings under a microscope, has revolutionized the detection of precancerous cervical changes, which are awkwardly called cervical intraepithelial neoplasia (CIN). CIN is graded from a scale of 1 to 3, with 3 being the most advanced. Women are encouraged to get Pap smear screenings on a routine basis. The Pap smear guidelines are pretty complicated, and I won’t cover the nitty gritty. The take home point is that when a cervix has advanced CIN, the affected part of the cervix should be removed (cervical conization, LEEP procedure).  

Other oncogenic infections include Epstein-Barr virus (Burkitt’s lymphoma, Hodgkin lymphoma, nasopharyngeal cancer, primary CNS lymphoma), hepatitis B & hepatitis C (hepatocellular carcinoma), H. pylori (gastric adenocarcinoma, MALT lymphoma), HTLV-1 (adult T cell lymphoma), chinese liver fluke (cholangiocarcinoma), Schistosoma haematobium (squamous bladder cancer).