What is even more striking is that in both the U.S. and the Northern Territory of Australia, secondary infertility accounts for about 70% of all infertility, whereas in sub-Saharan Africa, secondary infertility accounts for about 90% of all infertility.

What might explain the differences between Australia and Africa, both regions with populations at high risk for STDs and PID?

First, the earlier age at first childbirth in Africa accounts for lower rates of primary infertility.
Second, the HIV endemic and high rates of STDs in some regions of Africa may cause more PID and PID related secondary infertility.