The word virus, Ken Alibek explains (in Biohazard), comes from the Latin term for poison:
Viruses are invisible under most microscopes and hundreds of thousands of times smaller than a grain of sand. Their existence was unsuspected until Dmitry Ivanovsky, a Russian microbiologist, discovered them in the late nineteenth century while investigating an outbreak of mosaic disease in tobacco plants. Ivanovsky found that the mysterious agent responsible for this disease was able to pass through filters that otherwise blocked bacteria. Over half a century would pass before the first virus was seen and identified under an electron microscope, but Ivanovsky’s discovery launched a new field of research into infectious diseases.
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The immune system works on many levels at once, like an army with scouts and infantrymen, naval and air power, a sophisticated information network, and a carefully delineated command structure. Some cells are responsible for surveillance, others for coordinating information; some focus on local maneuvers while still others direct more general attacks. Immunologists distinguish between specific and nonspecific immunological reactions. Specific or acquired immune responses depend on memory cells, which store information about previous invaders and thus play a significant role in conferring immunity.
Among the most important agents in the immune system are T cells. They act as scouts, circulating through the bloodstream and moving into lymph nodes, on the lookout for foreign substances. As soon as a virus enters the bloodstream and infects its first cell, it will be recognized by T cells, which immediately activate, replicating themselves and sending out signals, calling for the formation of antibodies and attracting them to the site of infection. Antibodies are like ground troops. They are particularly effective at attacking viruses and bacteria that are still coursing through the system, before they have infiltrated target cells.
Within seconds of infection, defensive proteins and inflammatory agents are released, which activate natural killer cells and lead them to the site of infection. Interferon, one of the most powerful antiviral agents, degrades viral RNA, slows down protein synthesis, and inhibits viral reproduction in infected cells.
By the end of the first week or the beginning of the second, the body will in many cases have developed virus-specific antibodies, which sometimes seek to neutralize the virus by binding to its surface and preventing it from penetrating into new cells. But viruses are adept and mutate quickly. Countless are now capable of inhibiting and neutralizing the body’s natural defenses, rendering their resistance ineffective.
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More than one hundred different viruses have been identified as causes of the common cold.