Human T lymphotropic virus type‐1 (HTLV‐1) is a well‐known human oncovirus, associated with two life‐threatening diseases, adult T cell leukaemia/lymphoma (ATL) and HTLV‐1 associated myelopathy/tropical spastic paraparesis (HAM/TSP). The study of this oncogenic virus is significant from two different aspects. First, HTLV‐1 can be considered as a neglected public health problem, which may spread slowly worldwide. Second, the incidence of HTLV‐1 associated diseases due to oncogenic effects and deterioration of the immune system towards autoimmune diseases are not fully understood. Furthermore, knowledge about viral routes of transmission is important for considering potential interventions, treatments or vaccines in endemic regions. In this review, novel characteristics of HTLV‐1, such as the unusual infectivity of virions through the virological synapse, are discussed in the context of the HTLV‐1 associated diseases (ATL and HAM/TSP).