Endometriosis is an a condition where the tissue that normally is found inside the uterine ends up growing and proliferating outside the uterus. It affects an estimated 1 out of 10 women in the world. However, that estimate is subject to dispute because the only sure way to tell if someone has endometriosis is to look surgically.
Every month, in a normally cycling woman, the lining of the uterus builds up in preparation of nourishing any fertilized egg (embryo) that comes into the uterus. If no viable embryo shows up, then the lining is shed in a process known as menstruation. Most of the blood and tissue will flow out the cervix and out of the body. However, in every woman who has open tubes, some of the blood and tissue will flow backwards through the tubes and into the abdominal cavity.
Under normal circumstances, the body's immune system will clean up this material. However, in an abnormal circumstance, the internal buildup of menstrual debris overwhelms the body's clean-up capacity and results in the tissue building up, taking hold and digging in. These endometriosis implants can cause intense pain (but not always) and they can contribute to infertility. Some of the ways that endometriosis can cause infertility is by scarring the tubes and scarring the areas outside the tubes. The process also creates a hostile inflammatory environment so that eggs die faster, even if they somehow traverse the obstacle course created by the scarring and make it into the tubes.
Solutions to address the pain include surgery and medications. Solutions to address the infertility include IVF (to completely bypass the endometriosis) or a combination of fertility drugs and insemination. The principle behind using fertility drugs and insemination is related to flooding the system with excess eggs and sperm. Even though the eggs are still subject to a hostile tangled environment there is an advantage to having multiple eggs. There develops a higher chance that even though most eggs will be trapped and die, perhaps a single lucky one can elude the endometriosis and make it to safety to be fertilized and form a baby.