Poxviruses are the largest and most complex viruses that occur in humans, birds, animals, and insects. These viruses, which belong to family Poxviridae, include a large group of DNA viruses that are morphologically similar and share a common nucleocapsid protein.
The family Poxviridae, based on whether they infect insect or vertebrate hosts, has been classified into two subfamilies, Chordopoxvirinae and Entomopoxvirinae. The subfamily Chordopoxvirinae contains viruses that infect vertebrate hosts and include eight genera, of which at least four genera cause diseases in humans. These genera are as follows:
Orthopoxvirus: The genusOrthopoxvirusincludes the pox-viruses of mammals, such as smallpox (variola), vaccinia, monkeypox, cowpox, buffalopox, rabbitpox, mousepox, and camelpox viruses.
Parapoxviruses: These include the viruses of ungulates thatmay cause occasional infections in humans. These viruses are orf viruses, pseudocowpox virus, deerpox viruses, and bovine papular stomatitis virus.
Yatapoxviruses: These include tanapox viruses and yabapoxviruses that are found mainly in Africa.
Molluscipoxviruses: These include molluscum contagiosumvirus.
Most of the poxviruses that cause diseases in humans belong to the genera Orthopoxvirus and Parapoxvirus. Smallpox and molluscum contagiosum viruses are specific human patho-gens, whereas other poxviruses cause rare zoonotic infections in humans. The poxviruses causing infections in humans are listed in Table 55-1.