Some scientists think it's caused by an infectious agent, such as a virus.

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The infectious agent takes at least six to eight years to cause symptoms in cows, meaning that the infected cow was likely spreading the disease during that time.

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"Currently, we have identified no infectious agent common to all of the cases, " the statement said, adding that no evidence exists that the cases were caused by exposure to chemical or biological weapons, SARS or environmental toxins.

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The only way to eliminate any risk of cross-infection in the aircraft cabin -- or the rapid worldwide spread of an infectious agent -- is to prevent would-be passengers who are either substantially exposed to or carrying transmissible infections from flying.

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The Ebola virus is a highly infectious, often fatal agent spread through direct contact with bodily fluids.

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The deaths have stoked heightened fear of the virus, a highly infectious, often fatal agent spread through direct contact with bodily fluids.

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