I had watched Children of men recently, I'm confused in one aspect
What caused the infertility?
The source/cause of the infertility is never explained in the movie and very likely unknown. This, in many ways, is what the Human Project is trying to ascertain in order to find a cure.
While the women are the ones said to be infertile in the film, it's the men who are infertile (zero sperm count) in the original P.D. James novel.