Viruses are a form of life that possess genes but do not have a cellular structure. Viruses do not have their own metabolism, and they require a host cell to make new products; therefore, they cannot naturally reproduce outside a host cell. The objective of this paper is to present the basic practical clinical roles of viruses in patients with hematological diseases including malignancies and non-malignan- cies, as well as those undergoing hematopoietic cell transplantation (HCT), with the focus on herpesviruses causing latent infections in severely immunocompromised patients. From the hematologist point of view, viruses can play a major role in four conditions: causing infections; causing lymphoproliferations and/or malignancies; causing (pan)cytopenia; and used as vectors in treatment (e.g., gene therapy, CAR-T cells). Taking into account the role of viruses in hematology, infection is the most frequent condition. Among DNA viruses, the highest morbidity potential for human is expressed by Herpesviridiae (herpesviruses), Adenoviridae (adenovirus; ADV), Polyomavirus (BKV, JCV), and Bocavirus. RNA viruses can play a role in pathogenesis of different clinical conditions and diseases: lymphoproliferative disorders and malignancy, possibly causing NHL, AML, MDS, and others (HCV, HIV, and others); pancytopenia and aplastic anemia (HIV, HCV, Dengue virus); respiratory infections (community-acquired respiratory virus infections; CARV) caused by Orthomyxoviruses (e.g. influenza A/B), Paramyxoviruses (e.g. human parainfluenza virus PIV-1, -2, -3, and -4; respiratory syncytial virus RSV-A and -B), picornaviruses (e.g., human rhinovirus), coronaviruses (e.g., human coronavirus), Pneumoviridiae (e.g., human metapneumovirus), and potentially other viruses.