Viruses are particles that function on the border of living organisms and chemical compounds. They have the ability to create crystal structures, which is typical for inanimate matter, and on the other hand, can multiply and carry diseases, including upper respiratory tract infections. Most viruses are much smaller than bacteria, making them impossible to observe under a standard microscope. They consist of nucleic acids (DNA or RNA) and a capsid (a protein shell containing the genetic material of the virus). Viruses do not have their own organs to reproduce. They are intracellular parasites, which means that cells of other organisms are used for reproduction. Viruses place their genetic information inside the base cell and force it to reproduce the viruses (replication). The protein shell protects the nucleic acid from various physical and chemical influences and prevents cell enzymes from penetrating it, thus preventing its breakdown (protective function). Also, the capsid has a receptor complementary to the receptor of the infected cell – viruses strike a strictly defined circle of hosts (determinant function).
Structure of viruses
The simplest viruses are a nucleoprotein that consists of nucleic acid (RNA or DNA) and a protein shell capsid. More complex viruses have an additional lipid shell. There is a type of virus, bacteriophages, which have a special structure that allows them to introduce their genome into bacterial cells. Bacteriophages have a body consisting of a head with the genome, a tail (a tube that transports the genome into the cell) and spurs. Viruses can enter the cell by dissolving the cell shell or by immersing the shell fragments together with the virus into the cytoplasm or together with pinocytosis bubbles. Once in the cell, the virus begins to reproduce with a cell that synthesizes the virus’s DNA or RNA. The cell is damaged and then dies, and the viruses are able to infect other cells. In this way, the virus can exist and multiply almost endlessly. There are many different viruses that cause dangerous diseases: influenza, hepatitis, AIDS and others. The most dangerous and undiscovered is the human immunodeficiency virus (HIV), which causes acquired human immunodeficiency syndrome (AIDS), which enters the body through sexual contact or blood. This virus affects the human immune cells, making them vulnerable to any disease, which can cause a person to die even from a cold. Viruses affecting the human body and animals have the ability to mutate very quickly to reproduce. This fact makes the viral diseases extremely resistant to treatment.
History of virus research
As data on infectious diseases of various organisms accumulated, it became clear that not all of them were caused by pathogens known at the time – bacteria, protist or microscopic fungi. In particular, Louis Pasteur could not find the agent causing rabies and assumed that this pathogen was too small to be seen in a microscope. In 1884, French microbiologist Charles Chamberlain invented a filter (now known as Chamberlain filter or Chamberlain-Paster filter) whose pores are smaller than those of bacteria. With this filter, bacteria can be completely removed from the solution. In 1892, Russian biologist Dmitry Ivanovsky used it to study a species now known as the tobacco mosaic virus. His experiments showed that the extract of shredded leaves of infected tobacco plants retains infectious properties even after filtering. Ivanovsky suggested that the infection could be caused by a toxin released by bacteria, but he did not develop this idea. At that time, it was believed that any infectious agent can be isolated on the filter and grown in a nutrient medium – this is one of the postulates of microbial theory of disease.
Ivanovsky in an optical microscope observed in the infected cells of plants crystal-like bodies, which in the modern sense were clusters of viruses, later they were called “Ivanovsky crystals”. In 1898, the Dutch microbiologist Martin Beierink repeated Ivanovsky’s experiments and concluded that the infectious material passed through the filter was nothing but a new form of infectious agents. The agent multiplied only in dividing cells, but his experiments did not reveal that it was a particle. Beyerink called it Contagium vivum fluidum (literally lat. soluble live microbe) and reintroduced the word “virus”. He claimed that the virus was liquid in nature. This theory was later disproved by Wendell Stanley who proved that viruses are particles. In the same year, Friedrich Löffler and Paul Frosch discovered the first animal virus, the foot-and-mouth disease causative agent (afterovirus), by passing it through a similar filter.