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

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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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It is a general antiviral agent that does not specifically target the hepatitis C virus at all.

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He said an FBI agent told him that his company was the victim of a new virus and an unknown perpetrator.

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