The production or quality of sperm may be affected by congenital and genetic abnormalities, injuries to the genital tract, heat, age, sperm agglutination, acute and chronic infection (often sexually transmissible infections), malnutrition, previous surgery, allergies, chronic illness, environmental or occupational factors (such as radiation), varicocele, or certain medications. Among these medications are Tagamet, used in ulcer treatment; drugs used for treating cancer; and some antibiotics (especially those used to treat tuberculosis). Also heavy smoking of marijuana and smoking generally, alcoholism and stress may result in impotence or inability to ejaculate.

Varicocele, a varicose enlargement of the veins of the spermatic cord, is a potentially curable cause of male infertility. While this condition occurs in many men with normal fertility, it has been found to be present in as many as forty percent of infertile men. Half of all men with varicoceles have decreased sperm count or sperm motility or other changes in the semen analysis. Theories of the cause of these changes include heat, pressure and toxic substances from the dilated vessels.

Permanent or temporary damage to the male testis can occur as a result of a genital infection or a systemic infection. Gonorrhea may do enough damage to the male genital tract to result temporarily in a marked decrease in the sperm count. Mumps in an adult male may involve one or both testicles and may cause severe testicular damage. Fortunately, usually only one testicle suffers severe impairment and the sperm count, though possibly reduced, is usually compatible with fertility. Any systemic viral or bacterial infection may cause a temporary depression in the sperm count.

Because many of the infertility tests for women are more complicated and involve more risk than those for men, infertility testing often begins with the male. A semen analysis is a simple test that can provide a great deal of information. The male is asked to submit a recently ejaculated semen specimen to the physician or laboratory. This specimen is then examined microscopically to determine sperm count, their size and shape and if they are able to move normally. There is no sharp line of demarcation between fertility and sterility in the sperm count. Counts of less than twenty to forty million per cubic centimeter are often correlated with decreased fertility, although men with counts of five to ten million have fathered children. A high percentage of sperm with abnormal shape, size, or decreased motility is also correlated with decreased fertility.

The semen can be analyzed also for antibodies and cultured for various infections. The hormone levels in the man's blood are also measured to make sure his hypothalamus and pituitary glands are functioning normally.

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