Papillomaviridae is a taxonomic family of non-enveloped DNA viruses, collectively known as papillomaviruses. There are several hundred species, or types, of papillomaviruses that have been identified. Most infections are asymptomatic or cause small benign tumors. Types 16 and 18 carry risks of becoming cancerous.
They replicate in the basal layer of the body surface tissues. Typically they infect the skin but can infect a particular body surface. HPV type 1 tends to infect the soles of the feet while type 2 attacks the palms of the hands.
They were first identified in the early 20th century when it was shown that skin warts could be transmitted between individuals by a filterable infectious agent. Francis Peyton Rous, in 1935, was the first to demonstrate that the virus could cause cancer in mammals. They tend to be highly adapted to replication in a single animal species. Bovine Papillomavirus type 1 has been documented to be inter-species transmissible.
Its evolution has been slow compared to other virus types. The papillomavirus genome is genetically stable double-stranded DNA. It is believed to have co-evolved with a particular species of host animal over many years. HPV-13 varies little in different human populations. It is not clear if this is due to inter-species transmission or if the virus has changed very little in six or so million years.
They replicate exclusively in keratinocytes, the outermost layers of the skin. Papillomaviruses gain access to keratinocyte stem cells through small wounds in the skin. The keratinocyte expresses E1 and E2 proteins which replicate and maintain viral DNA as a circular episome. Some types can cause cancer in the epithelial tissues they inhabit although that is not typical.