CD4 and syphilis: Syphilitic ulcers can facilitate HIV transmission, and HIV can modify the clinical course of syphilis. It also increased concentrations of HIV in blood plasma and decreased CD4+ cells. This can lead to atypical presentations and an increased risk of neurosyphilis. The secondary systemic stage can heighten immune activation in host cells, promoting increased replication of HIV. Neurosyphilis can occur at any stage of syphilis.