By Rae Paoletta

Endometriosis is a disorder that affects 176 million women worldwide. Yet, many people don't actually know what it is.

The condition occurs when "tissue similar to the endometrium (the lining of the uterus) is found outside the uterus on other parts of the body," according to the Endometriosis Foundation of America.

This abnormality in the uterine lining has painful ramifications, like heavy periods, "killer cramps," pain during sex, and even infertility. It's anything but easy.

Over the last year, celebrities like Halsey and Lena Dunham have opened up about their struggle with the condition — and they're far from alone.

In just the United States, endometriosis affects 1 in 10 girls; but that doesn't stop them from living fulfilling lives. Here are just some of their brave stories: