The normal reproduction process requires healthy interaction between the female and male reproductive tracts. The woman ovulates and releases an egg from her ovaries to travel through the Fallopian tube all the way to her uterus. The male produces and releases sperm. Both egg and sperm normally meet in the  Fallopian tube, where fertilization takes place. Infertility occurs when something in this pattern happens in a wrong way or does not happen at all. Common causes of infertility include:

Age - women become less fertile as they age into their fifth decade of life (age 40-49 years). The older men get, the less levels of testosterone are produced that significantly changes the volume and concentration of sperm.

Pelvic Inflammatory Disease (PID) - PID damages Fallopian tubes where an egg travels through from ovaries to uterus. The condition is commonly caused by Sexually Transmitted Diseases such as gonorrhea and Chlamydia. 

Endometriosis -  the condition takes place when bits of the tissue that lines the uterus grow on other pelvic organs, for example, the ovaries or fallopian tube. A woman may be at higher risk for developing it if she has a family history of the disease.

Tobacco and Drug Use -nicotine and drugs block and hinder the production of sperm and decreases the size of a man's testicles. As for women, they change the cervical mucus, hence affecting the way sperm reaches an egg. 

Unhealthy Weight - extreme weight loss or gain is one of the biggest threats when it comes to infertility. Obesity and anorexia can create problems connected to menstrual periods (no periods) and thyroid levels, thus disrupting normal ovulation. Excessive weight significantly decreases the quality of sperm.