What is a virus?

Viruses are the most common biological entities on Earth. At the same time, they are also the smallest: a drop of sea water, for example, already contains more than 10 million. 

Packet of DNA to hijack

A virus is a protein coat containing some genetic material. Sometimes there is a membrane around this protein coat (the so-called envelope). That’s it. In order to reproduce, and thus survive, it depends on a host. This can be a bacterium, plant or animal, for example. As soon as a virus encounters and enters a host, they hijack the cell as it were. They use the host cell’s machinery to make hundreds, if not thousands of copies of themselves. Eventually the host cell bursts open and the newly produced viruses spread further.

Little life or dead droplet?

Unlike all other types of microbes on Earth, viruses are not in the tree of life. Viruses do not consist of a cell, such as a bacterium, alga or slipper animal. They have no organelles, cannot move by themselves, and have no metabolism of their own. Some scientists therefore believe that they cannot really be called "alive". Other scientists believe however that viruses are related to everything else on Earth, because they also have DNA or RNA for example, and are subject to evolution.

Good versus bad

We deal with viruses every day. They occur all around us. We don't notice anything of the vast majority of them, but most of us will recognize the consequences of meeting viruses. Think of a common cold caused by rhinoviruses, or a sometimes more serious flu caused by influenza viruses. Many notorious diseases, such as AIDS, or the currently eradicated smallpox, are also caused by a virus.

But viruses are not only bad for us. Many viruses only infect bacteria and keep the number of harmful bacteria low. These types of viruses are called bacteriophages, or phages for short. A phage is species-specific. This means that it only parasitizes one specific type of bacteria. Just like bacteria, there are many different types of phages. Bacteria cannot become resistant to their phage. When bacteria adapt under the pressure of phages, the phages change just as quickly: an evolutionary arms race. The human microbiome contains multiple useful phages.