Well, we all have learned a lot more about viruses in the last two years with the Covid-19 global pandemic. But the viruses that infect humans are only a small fraction of the viruses that you find in the world. The greatest abundance of viruses are those that infect bacteria and that are sometimes called phage.
Anyway, let’s go back in time, to the 19th century, when two European scientists, Robert Koch and Louis Pasteur, discovered that some diseases in animals and plants were caused by germs. Thanks to them, virology (a new branch of science) was born.
Viruses are the smallest of all the microbes. They are very strange: they don’t have a precise shape: They have a head, protected by a shell, a tail and some fibers. Inside they contain DNA or RNA, some proteins and lipids. Their name comes from a Latin word that means “poison” and, in fact, a virus attack can cause a disease. It can be a simple flu or something more serious.
Viruses only exist to make more viruses. When viruses want to replicate, they attack a cell and start living in it. This cell is called “host cell”. The virus particle attaches to the host cell before penetrating it. The virus then uses the host cell’s machinery to replicate its own genetic material. Once replication has been completed the virus particles leave the host by either budding or bursting out of the cell (lysis). In the first case, as the newly formed viral particle pushes against the host cell’s plasma membrane a portion adheres to it. The plasma membrane envelops the virus and becomes the viral envelope. The virus is released from the cell. This process slowly uses up the host’s cell membrane and usually leads to cell death. In the case of Lysis, the virus particles burst out of the host cell into the extracellular space resulting in the death of the host cell. Once the virus has escaped from the host cell it is ready to enter a new cell and multiply.
Today, more than 2,000 types of viruses have been identified.