Infection in the narrow sense is a parasitic relationship between a microorganism and a host where the causative agent multiplies and leads to a general inflammatory response. Infection can be exogenous or endogenous; exogenous is caused by microorganisms from the environment, and endogenous is caused by a weakening of the host's resistance, so microorganisms already living in his body cause tissue damage or a reaction of the organism. The host fights infection using its immune system. Particularly at risk are immunocompromised children, which primarily include children with acute lymphocytic leukemia (ALL), as it is the most common malignant disease of childhood. Children with ALL are prone to infection due to the nature of the disease itself and additionally the treatment options. In Croatia and many other countries, the German BFM (BerlinFrankfurt-Munster) protocols are used in the treatment of ALL, which determine therapeutic regimens and define standard, medium and high risk groups. The protocol consists of corticosteroids and a combination of multiple cytostatics. This combination of drugs can cause life-threatening conditions when it comes to infection. In some patients, primarily from the highrisk group and in the event of relapse of the disease, it is necessary to do HSCT (hematopoietic stem cell transplantation), which additionally immunocompresses the already exhausted organism. Transplanted patients also take immunosuppressants to prevent graft rejection. The most common causes of infections in immunocompromised children are bacteria, viruses and fungi, protozoa are less common. Clinical pictures may be atypical or asymptomatic due to a weakened immune response and the outcome uncertain especially if they are resistant agents. It is extremely important to act preventively in order to prevent the infection, and if it occurs, to act immediately by all available means. It is a special responsibility and challenge for the entire team, and especially for the nurses, to take care of an immunocompressed child. It is necessary to know the possible critical points for each patient, monitor all changes in the child, apply the rules of aseptic mode, constantly upgrade their knowledge, and educate loved ones about all available measures and rules of conduct to help reduce possible sources of infection.