Simplify the following text for a Grade 9 native English-speaking person.
Original: The Human T-lymphotropic virus Type I ( HTLV-1 ) is a human RNA retrovirus that causes T-cell leukemia and T-cell lymphoma in adults and may also be involved in certain demyelinating diseases , including tropical spastic paraparesis .
Simplified: It is a kind of retrovirus . Human T-lymphotropic virus causes leukemia as well as other diseases .