Viruses are the freeloading vampires of the body. Not a plant, animal, or bacterium, these infectious agents are unable to live on their own. Instead, they hijack other cells to help them reproduce and carry out metabolic activities.

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First identified in 1892 by the Russian scientist Dmitry I. Ivanovsky (1864–1920), viruses get their name from the Latin word meaning “poison” and “slimy liquid.” Today, scientists have identified more than 5,000 different types of viruses. Some of them, such as those that cause the common cold and flu, multiply rapidly and kill the host cell, while others, such as the genital herpes virus, can lie dormant in the body for years. Still others, such as the one that leads to AIDS, are slow viruses, meaning that they remain in the cells and replicate slowly over years.

Essentially, viruses consist of nucleic acids—either DNA or RNA—surrounded by a protein shell, or capsid. Most viruses are shaped as either rods or spherical, 20-sided polygons. Because they lack enzymes for energy production and ribosomes for protein synthesis and reproduction, viruses latch onto a host cell. Like a needle, they inject their genetic material into the new cell. At that point, the viral nucleic acid takes over the cellular machinery, producing new virus particles. Often, this process destroys the host cell. This releases new viruses to infect other cells.

Because viruses are embedded in the cells, they’re much more difficult to kill than bacteria. Some antiviral medications attack viruses, while vaccines are also effective in stimulating the immune system to produce white blood cells that target specific viruses.

ADDITIONAL FACTS

  1. Viruses range from 20 to 400 nanometers in size. Only the largest viruses can be seen under a powerful microscope on the highest resolution.
  2. Some experts theorize that a virus is to blame for the extinction of dinosaurs.
  3. The viruses that cause the common cold can live on surfaces outside the body for a few days.