HTLV-1, identified as the first human oncogenic retrovirus 30 years ago, is not widespread but is present globally, with high endemic areas in Southwest Japan, sub-Saharan Africa, South America, the Caribbean, and parts of the Middle East and Oceania. The distribution is likely due to a founder effect in some populations with persistent high transmission rates. HTLV-1 prevalence increases with age, especially among women in high-endemic regions. Transmission occurs through mother-to-child, sexual contact, and contaminated blood products. Previous estimates suggested 10–20 million infected individuals, but recent data indicate 5–10 million, based on 1.5 billion individuals from known endemic areas. However, accurate estimates for regions like China, India, and East Africa are lacking. HTLV-1 is mainly found in specific clusters, with low prevalence in surrounding areas. The virus has a high genetic stability, leading to distinct molecular subtypes. HTLV-1 is endemic in Japan, the Caribbean, parts of South America, and Africa, with Romania being the only endemic area in Europe. In Europe, most infected individuals are immigrants from high-endemic areas. In Africa, HTLV-1 is widespread, with varying prevalence rates across regions. In the Americas, HTLV-1 is rare in North America but present in South America, particularly in specific populations. The global prevalence is estimated to be around 5–10 million, though actual numbers may be higher due to underreporting.HTLV-1, identified as the first human oncogenic retrovirus 30 years ago, is not widespread but is present globally, with high endemic areas in Southwest Japan, sub-Saharan Africa, South America, the Caribbean, and parts of the Middle East and Oceania. The distribution is likely due to a founder effect in some populations with persistent high transmission rates. HTLV-1 prevalence increases with age, especially among women in high-endemic regions. Transmission occurs through mother-to-child, sexual contact, and contaminated blood products. Previous estimates suggested 10–20 million infected individuals, but recent data indicate 5–10 million, based on 1.5 billion individuals from known endemic areas. However, accurate estimates for regions like China, India, and East Africa are lacking. HTLV-1 is mainly found in specific clusters, with low prevalence in surrounding areas. The virus has a high genetic stability, leading to distinct molecular subtypes. HTLV-1 is endemic in Japan, the Caribbean, parts of South America, and Africa, with Romania being the only endemic area in Europe. In Europe, most infected individuals are immigrants from high-endemic areas. In Africa, HTLV-1 is widespread, with varying prevalence rates across regions. In the Americas, HTLV-1 is rare in North America but present in South America, particularly in specific populations. The global prevalence is estimated to be around 5–10 million, though actual numbers may be higher due to underreporting.