FIV compromises the immune system of cats by infecting many cell types, including CD4+ and CD8+ T lymphocytes, B lymphocytes, and macrophages. FIV can be tolerated well by cats, but can eventually lead to debilitation of the immune system in its feline hosts by the infection and exhaustion of T-helper (CD4+) cells. The rate of occurrence in cats is at less than 5%;[citation needed] in humans with HIV, this percentage is estimated to be over 50%.