Viruses are tiny infectious agents that rely on living cells to multiply. They may use an animal, plant, or bacteria host to survive and reproduce. As such, there is some debate as to whether or not viruses should be considered living organisms. A virus that is outside of a host cell is known as a virion.

Not only are viruses microscopic, they are smaller than many other microbes, such as bacteria. Most viruses are only 20–400 nanometers in diameter, whereas human egg cells, for example, are about 120 micrometers in diameter, and the E. coli bacteria has a diameter of around 1 micrometer. Viruses are so small that they are best viewed using an electron microscope, which is how they were first visualized in the 1940s.

Viruses generally come in two forms: rods or spheres. However, bacteriophages (viruses that infect bacteria) have a unique shape, with a geometric head and filamentous tail fibers. No matter the shape, all viruses consist of genetic material (DNA or RNA) and have an outer protein shell, known as a capsid.

There are two processes used by viruses to replicate: the lytic cycle and lysogenic cycle. Some viruses reproduce using both methods, while others only use the lytic cycle. In the lytic cycle, the virus attaches to the host cell and injects its DNA. Using the host’s cellular metabolism, the viral DNA begins to replicate and form proteins. Then fully formed viruses assemble. These viruses break, or lyse, the cell and spread to other cells to continue the cycle.

Like the lytic cycle, in the lysogenic cycle the virus attaches to the host cell and injects its DNA. From there, the viral DNA gets incorporated into the host’s DNA and the host’s cells. Each time the host’s cells go through replication, the virus’s DNA gets replicated as well, spreading its genetic information throughout the host without having to lyse the infected cells.

In humans, viruses can cause many diseases. For example, the flu is caused by the influenza virus. Typically, viruses cause an immune response in the host, and this kills the virus. However, some viruses are not successfully treated by the immune system, such as human immunodeficiency virus, or HIV. This leads to a more chronic infection that is difficult or impossible to cure; often only the symptoms can be treated.

Unlike bacterial infections, antibiotics are ineffective at treating viral infections. Viral infections are best prevented by vaccines, though antiviral drugs can treat some viral infections. Most antiviral drugs work by interfering with viral replication. Some of these drugs stop DNA synthesis, preventing the virus from replicating

Although viruses can have devastating health consequences, they also have important technological applications. Viruses are particularly vital to gene therapy. Because some viruses incorporate their DNA into host DNA, they can be genetically modified to carry genes that would benefit the host. Some viruses can even be engineered to reproduce in cancer cells and trigger the immune system to kill those harmful cells. Although this is still an emerging field of research, it gives viruses the potential to one day do more good than harm.