Viral diseases or infections are caused by viruses and can appear as local inflammation or as a systemic disease. Very often, after the incubation period, they manifest as a short-term illness with general symptoms. Viruses can stay in the body in a latent form for years without major consequences or cause a lifelong chronic infection and multiply constantly, despite the patient's defense mechanisms. An important role in the development of a viral infection is played by the sensitivity of the cell to the virus. By multiplying inside the cell, it can interfere with its metabolism and cause its death, it can change the properties of the cell and thus lead to the change of the cell from normal to malignant, or it can multiply without disrupting the function of the cell. Viruses can cause damage to many organic systems of the peripheral nervous system, digestive system, liver, pancreas, heart and blood flow, immune system, skin, mucous membrane, urinary system, sexual organs. Many viruses have oncogenic properties and are associated with the formation of some tumors (eg human papillomavirus, Epstein-Barr virus, hepatitis B and C virus).