Infertility affects approximately 15-20% of all couples. Male infertility factors contribute to approximately 50% of all infertility cases, and male infertility alone accounts for approximately one-third of all infertility cases.

Male infertility is diagnosed when the sperms are not enough or they are abnormal.

There are many causes of infertility among men.

• Idiopathic oligospermia: This is probably the major reason for male infertility. It is diagnosed when all other tests were normal and is another way of saying that no cause is found. 
• Varicocele: A varicocele is an enlarged vein in the scrotum. This vein causes an elevated temperature which then affects the production of sperms. A varicocele is one of the most common and readily treatable causes of male infertility. Others say that varicocele is also common among fertile men, and question the connection with infertility and the need for treatment. Large varicoceles that go untreated can cause permanent damage to the testicles. This can lead to testicular failure or atrophy.
• Cancer treatment: Chemotherapy and radiation can cause abnormal sperm or sterility.
• DES (diethylstilbestrol) exposure: Synthetic estrogen used in the 50s and 60s used by women to prevent miscarriage. Can cause low sperm counts, decreased sperm motility, and abnormal sperm forms, small penises, undescended testicles (risk factor for testicular cancer), abnormal testicles.
• Hormonal imbalances: Hormone problems affecting sperm count include thyroid problems, low testosterone levels, elevated FSH, and excess prolactin (see next entry).
• Hyperprolactinemia (excess prolactin): can inhibit GnRH, resulting in lower LH and testosterone. Also low FSH.
• Immune problems: Both men and women can have immune reactions to sperm. There is a lot of controversy about how prevalent this is. Immune reactions to sperm in the man (autoimmune) can be a problem post-vasectomy, but may also have other causes. Anti-sperm antibodies in the male are often indicated by hyperviscosity which may inhibit forward progression. In mild cases, anti-sperm antibodies in the male or female (alloimmune) may be overcome by IUIs, for which the man will be asked to ejaculate into a cup with a special preparation in it. If IUI does not work, or if the problem is considered too severe, IVF may be necessary, with ICSI likely for male anti-sperm antibodies. Predisone, a steroid, may be given to the party producing the antibodies.
• Impotence: One of the less common causes. Note: impotence is a medical problem. There are a variety of medical causes that can contribute, including diabetes mellitus, certain required medications such as antidepressants, etc. Sexual advice from friends is generally not welcome. The drug Viagra, according to the manufacturer, does not appear to have any negative impact on sperm.
• Infection: Postpubertal mumps, and, occasionally, venereal diseases such as gonorrhea and chlamydia can harm male fertility. Also, recurrent infections such as prostatitis can lower sperm count and motility.
• Klinefelter's Syndrome: Men with Klinefelter's syndrome have two X chromosomes and one Y chromosome, rather than the normal one X and one Y. They are generally tall and thin, with small testicles.
• Lifestyle factors: These include factors which raise the temperature of the scrotum (such as the use of hot tubs or long baths), or harm sperm production. A variety of medicines and recreational drugs can decrease male fertility. These include alcohol, marijuana, cocaine, cigarettes, anabolic steroids, sulfasalazine, cimetidine (Tagamet, used for ulcers), nitrofurantoin (used for UTIs), anti-hypertensive drugs (specifically calcium channel blockers), aspirin, Dilantin (for epilepsy), colchicine, and antidepressants (note that some of these drugs should not be simply discontinued, because they may be required for other serious medical problems). Exposure to certain chemicals, such as lead and arsenic, and many types of paints or varnishes, can also adversely affect male fertility.
• Obstruction: Can occur at various points, blocking sperm from getting out. Treated surgically. Often may be easier to work around obstruction by doing MESA or TESA instead of trying to repair surgically.
• Prior surgery: The vas may be damaged during surgery fo hernia repair, orchiopexy, and even during varicocelectomy.
• Retrograde ejaculation: Can be caused by certain medications, surgeries, and nerve damage (for example, from diabetes mellitus). Sperm goes in the wrong direction and can be found in the urine.
• Sexual Dysfunction: Reported in up to 20% of infertile men. May include decreased sexual desire, inability to maintain an erection, and premature ejaculation. This could result from low testosterone or performance anxiety.
• Trauma to testicles: Injury to testicles, such as from being hit, followed by atrophy. May also be the result of having the mumps and develop bi-lateral orchitis.
• Undescended testicle: If the testicles do not descend during puberty, their body temperature may be too high, reducing quality and quantity of sperm production. Rare.
• Vasectomy reversal: Though vasectomies are meant as a permanent means of birth control, it turns out that they can often be reversed. However, it is easier to reverse them if not too much time has passed since the vasectomy. The more time has passed, the more likely it is that the man will have an immune reaction to his own sperm.