Virus

A virus is a minute particle consisting of genetic material (RNA or DNA) enclosed in a protein shell. Viruses cannot reproduce on their own. They must enter a living cell—a human cell, animal cell, plant cell, or bacterium—and then use that cell's machinery to produce new copies.

Most viruses measure hundreds of nanometres across, too small to be seen with an ordinary light microscope. For this reason they were unknown until the early twentieth century, when it was discovered that certain diseases could still spread even after passing through filters fine enough to stop bacteria.

Structure

Every virus has genetic material and a capsid—a protein shell made up of identical subunits arranged together. Some viruses also have a lipid envelope acquired from the cell they emerged from. This lipid envelope facilitates entry into a new cell, but it is also susceptible to soap and alcohol.

Replication

The general mechanism involves several steps: the virus attaches to a specific site on the cell surface (a receptor), then injects its genetic material inside. The cell begins to produce proteins and copies of the viral genome. The new components assemble together, then exit the cell—sometimes destroying the cell in the process, sometimes releasing gradually.

Viruses and disease

Many viruses cause diseases in humans, including measles, smallpox, influenza, HIV, and COVID-19. However, not all viruses cause illness, and extensive research has shown that large numbers of viruses exist in the environment, attacking bacteria (bacteriophages).

Prevention and treatment

Vaccination is the most effective means of preventing viral diseases. It prepares the immune system to recognize a virus before it spreads. Antibiotic drugs do not work against viruses, as they target bacteria. There are specific antiviral drugs that inhibit the stages of replication, but each works only against particular types.