SIVsm infection in mangabeys is accompanied by an early decline in and thereafter maintenance of CD4⁺ T-cell levels. Plasma viral load (A) and absolute number of CD4⁺ T cells (B) for 6 SIVsm-inoculated mangabeys. Three had thymuses (continuous lines; SM1, blue, □; SM2, turquoise, ▵; SM3, purple, ▿). Three underwent thymectomy (broken lines; SM4, green, ▴; SM5, black, •; SM6, red, ▪). Data are shown through 37 weeks of infection.