This 3D animation from The Human Atlas explains Infertility.

The testicles of adult men produce about half a billion sperm each day. By contrast, women generally release only one egg per month.

In order for fertilization and conception to occur, one of a man's many sperm must join the single egg released in a woman's oviduct or fallopian tube.

There are many causes of infertility, and often times the reason for a couple's inability to conceive is never determined. Some known causes of infertility in men include low sperm count or non-functioning sperm. Infertility in women can be caused by many factors; some known causes can include failure to ovulate or blocked fallopian tubes. Severe endometriosis, a disorder where pieces of the uterine lining stray and adhere to reproductive organs, is also a common cause of infertility in women.

Diagnosis and management by a physician may be successful in treating some couples with infertility. Some cases of infertility can be treated with medications or surgery. A physician can also discuss alternatives to natural conception, such as donor sperm, donor egg and adoption.