Paradoxically, the levels of properdin in the blood of patients with HIV infection or other acquired immunodeficiencies were not found to be low, as Dubos had speculated for other changing circumstances, but an X-linked inborn error of properdin identified in 1982 was found to underlie a selective predisposition to recurrent meningococcal disease (SI Appendix, Fig. S1). Here, CFP is linked to meningococcal infection.