Viruses are microscopic infectious agents made up of genetic material (DNA or RNA) and a coat of proteins. They are not independent living organisms, since they are not capable of independent life and reproduction outside the host cell. Instead, they capture and use the molecular machinery of the host cell to reproduce themselves.

Viruses are capable of infecting various types of living organisms, including bacteria, plants, animals and humans. They cause a variety of illnesses, including colds, flu, HIV, and others. The mode of transmission of viruses is diverse and can include droplet infection, contact with infected surfaces, as well as vector transmission through insects.

The fight against viruses includes the use of vaccines, antiviral drugs and hygiene measures. The study of viruses and their effects on organisms is an important area of medical and scientific work, as it helps to develop strategies for the control and treatment of viral infections.