<P> Four orthopoxviruses cause infection in humans: variola, vaccinia, cowpox, and monkeypox . Variola virus infects only humans in nature, although primates and other animals have been infected in a laboratory setting . Vaccinia, cowpox, and monkeypox viruses can infect both humans and other animals in nature . </P> <P> The life cycle of poxviruses is complicated by having multiple infectious forms, with differing mechanisms of cell entry . Poxviruses are unique among DNA viruses in that they replicate in the cytoplasm of the cell rather than in the nucleus . In order to replicate, poxviruses produce a variety of specialized proteins not produced by other DNA viruses, the most important of which is a viral - associated DNA - dependent RNA polymerase . </P> <P> Both enveloped and unenveloped virions are infectious . The viral envelope is made of modified Golgi membranes containing viral - specific polypeptides, including hemagglutinin . Infection with either variola major or variola minor confers immunity against the other . </P> <P> Transmission occurs through inhalation of airborne variola virus, usually droplets expressed from the oral, nasal, or pharyngeal mucosa of an infected person . It is transmitted from one person to another primarily through prolonged face - to - face contact with an infected person, usually within a distance of 6 feet (1.8 m), but can also be spread through direct contact with infected bodily fluids or contaminated objects (fomites) such as bedding or clothing . Rarely, smallpox has been spread by virus carried in the air in enclosed settings such as buildings, buses, and trains . The virus can cross the placenta, but the incidence of congenital smallpox is relatively low . Smallpox is not notably infectious in the prodromal period and viral shedding is usually delayed until the appearance of the rash, which is often accompanied by lesions in the mouth and pharynx . The virus can be transmitted throughout the course of the illness, but is most frequent during the first week of the rash, when most of the skin lesions are intact . Infectivity wanes in 7 to 10 days when scabs form over the lesions, but the infected person is contagious until the last smallpox scab falls off . </P>

Last reported case of smallpox in the us