Illustration of a T4-lymphocyte cell infected with Human T-lymphotropic Virus type I (HTLV-I) which causes leukaemia. The leukaemia is adult T-cell leukaemia-lymphoma. Here, the effect of the viral infection within the lymphocyte (white blood cell) has caused its surface to appear typically folded and bumpy. This is partly due to the effect of the viruses budding out from the cell membrane. The virus has instructed the T4-cell to reproduce more viruses. This leukaemia affects the elderly; HTLV- I virus remains latent for years; when active, abnormal T4-cells increase in the blood and the immune system cannot fight secondary infections.