The American Urological Association Foundation (AUAF) has defined male infertility as any condition in which the man adversely affects the chances of initiating a pregnancy with his female partner.
The American Academy of Family Physicians (AAFP) reported about one-third of infertility cases are caused by male factors alone.
To be fertile, a man must deliver an adequate quantity of normal sperm to a woman's vagina, and the sperm must fertilize the egg according to Merck Manuals.
Mayo Clinic listed male infertility symptoms to include the inability to conceive a child; problems with sexual functions such as erectile dysfunction; pain, swelling or a lump in the testicle area; or signs of a chromosomal or hormonal abnormality.
The University of Maryland Medical Center (UMMC) said more than 90 percent of male infertility cases are due to sperm abnormalities. These include low sperm count, poor sperm motility and abnormal sperm shape.
Numerous conditions may lead to male infertility and sperm abnormalities. The most common is a varicocele. AAFP described a varicocele as enlarged veins on one or both sides of the scrotum. These veins make scrotum warmer inside and can reduce sperm production by the testicle on the same side.
Other causes of male infertility include a blockage that prevents delivery of sperm, disease, illness, injury, hormonal imbalances, genetic defects, certain medications; undescended testicles, and infections, like sexually transmitted diseases.
AUAF said another cause is retrograde ejaculation. This occurs when semen pushes backwards into the bladder instead of out the penis. An About.com article added that problems with erections or other sexual functions can cause male infertility.
Then there are environmental factors. Merck Manuals said overexposure to heat, heavy metals, industrial or environmental toxins, and pesticides can reduce sperm production. Mayo Clinic added that exposure to radiation or X-rays and prolonged bicycling are other possible causes.
In addition, there are lifestyle choices that may affect fertility.