Infection, including viral infection, still cause serious complication in the course of chemotherapy. Recognition of viral infections, monitoring, prophylaxis and treatment is aimed at reducing the number of infected patients, mitigating the cause of the disease and limiting deaths directly linked with infections in paediatric cancer patients. Viruses from the herpes group (HSV, VZV, EBV, CMV) are particularly dangerous. They can cause not only asymptomatic and local infectious but also general diseases and can reactivate, especially after BMT. Hepatoropic viruses (HBV, HCV) often lead to breaks in chemotherapy, while chronic viral hepatitis can lead to fibrosis, cirrhosis and even primary hepatocellular carcinoma. CMV, RSV, adenovirus influenza and parainfluenza virus cause diffuse interstitial pneumonitis and are also associated with a high rate of mortality. In this paper, we present the most frequency viral infection in children with malignant diseases, their methods of diagnosis and treatment.