Which substance is typically found in the blood if a person has or previously had an infecting virus?

The presence of antibodies in the blood indicates that a person has been exposed to an infecting virus and has mounted an immune response against it. Antibodies are specific proteins produced by the immune system in response to pathogens, like viruses. When a virus infects the body, the immune system recognizes the foreign invader and begins to produce antibodies that target that specific virus.

These antibodies can often be detected in blood tests, serving as markers to confirm past or present infections. Additionally, the presence of antibodies can indicate immunity to that particular virus, especially after vaccination or recovery from infection.

In contrast, while platelets play a crucial role in blood clotting and white blood cells are essential components of the immune system that assist in fighting infections, they do not specifically indicate a person's history of exposure to a virus. Antigens, which are substances that induce an immune response, are generally present on the virus itself rather than in response to infection, meaning they do not signify a past or present infection like antibodies do. Thus, antibodies are the key substance that confirms viral infection history.