It is known that infectious viruses, including the Epstein-Barr virus and cytomegalovirus, can be transmitted during allotransplants -- transplants involving individuals in the same species. Therefore, many worry that animal-to-human cell, tissue or organ transplants may make it easier for viruses to cross the species barrier. Below are some examples of viruses which are believed to have been transferred from animals to humans.

More commonly known as "mad cow" disease, bovine spongiform encephalopathy (BSE) is a progressive neurological disorder in cattle which is believed to have caused a fatal brain disease known as variant Creutzfeldt-Jakob disease (vCJD) in humans. The infectious agent linked to both BSE and vCJD is unknown -- current theories point towards a mutation in certain protein molecules known as prions. An outbreak of BSE in Great Britain in the 1990s appears to have been caused by the consumption of animal feed contaminated by infected sheep and/or cattle meat and bone meal.

The new variant of CJD has an incubation period of several years before symptoms emerge. It tends to affect younger people -- as of October 2000, the median age of death was 27.5 years. The first symptoms include serious psychological or sensory problems, followed by poor muscle coordination and mental confusion. The illness lasts for at least six months, with an average length of thirteen months.

The filovirus family of viruses, which include Ebola and the Marburg virus, cause severe hemorrhagic fever in humans and non-human primates. They are believed to be zoonotic viruses; however both their origins and their means of transmission, remain unknown.

The Marburg virus was first identified in 1967, when an outbreak occurred in Marburg and Frankfurt, Germany, as well as Belgrade. The first infected humans were laboratory workers who worked with African green monkeys in an effort to prepare polio vaccine. It is a rare disease in humans, although highly infectious.

The Ebola virus is named for a river in the Democratic Republic of Congo where it was first recognized in 1976. In 1995, a severe outbreak occurred in what was then called Zaire, in which 80% of the 316 people known to have the disease died. There are several subtypes of the Ebola virus -- one strain known as the Reston subtype was identified among monkeys imported from the Philippines to research facilities in Reston, VA. Several researchers became infected with this strain of the virus; however none became ill.

In May 1993, virologists in the southwestern U.S. discovered an outbreak of a new strain of hantavirus, a group of viruses transmitted to people by rodents that cause hemorrhagic fever and pneumonia. This particular strain, which was eventually named Sin Nombre Virus (SNV) caused seemingly healthy people to die suddenly of acute respiratory failure. The human disease caused by SNV is known as hantavirus pulmonary syndrome (HPS).

Researchers believe human immunodeficiency virus (HIV) most likely originated in non-human primates, probably chimpanzees. There are two types of HIV: HIV-1, which the predominant strain found in the United States, and HIV-2, primarily found in West Africa. HIV-2 is believed to have come from the sooty mangabey monkey. Scientists were unaware of the existence of the HIV virus in primates, until it crossed over into humans.

Some researchers believe that certain influenza viruses live in birds and are then passed to pigs, who may pass them on to humans. In 1918, an epidemic of influenza known as the "Spanish flu" was transferred from pigs to people and swept the globe, killing an estimated 20 to 40 million people. A 1997 flu outbreak in Hong Kong is thought to have incubated in chickens and spread directly to humans. Farmers slaughtered over a million chickens, which many experts believe likely prevented the disease from spreading further.

In 1999 an outbreak of a previously undiscovered virus occurred among pig farmers in Malaysia. Named after the village where it was first discovered, the Nipah virus caused a form of viral encephalitis, an inflammation of the brain. This virus is not believed to be spread by human to human contact -- most of the infected had direct contact with pigs. Over one hundred pig farmers died after experiencing fever, headache, dizziness and vomiting. More than two million pigs were slaughtered in an effort to contain the disease.

Sources: Cooper, David K.C. and Lanza, Robert P. Xeno: The Promise of Transplanting Animal Organs Into Humans. NY: Oxford UP, 2000.; Centers for Disease Control; Food and Drug Administration