According to the World Health Organization, a man with a sperm count of less than 20 million per milliliter of semen is at the risk of being infertile. The factors causing male infertility may be simple to complex in nature. Causes of male infertility can be classified as follows:

Chromosomal abnormalities - Abnormalities of the sex chromosomes contribute to the most complex type of male infertility. Since genetic defects cannot be treated, infertility in such cases cannot be treated. Classic examples of male infertility due to chromosomal abnormalities are Klinefelter’s syndrome and Noonan’s syndrome. In Klinefelter’s syndrome the chromosomal constitution is 47XXY i. e an extra X chromosome or a mosaic pattern of 46XY/47XXY. Males affected with Klinefelter’s syndrome have hypogonadism with azospermia. Estradiol levels in such individuals are greater than testosterone leading to feminization of physical characteristics like the appearance of gynecomastia. Another genetic abnormality of the sex chromosome is seen Noonan’s syndrome where the chromosomal constitution XO/XY. Such males suffer from hypogonadism and feminization of physical features. Other chromosomal abnormalities causing cystic fibrosis and kartageners syndrome also leads to impaired male fertility.
Hormonal : Deficiency of testosterone which is produced by the testes or the deficiency of the gonadotropin release hormone produced by the pituitary gland are also responsible for infertility in males.
Anatomic: Cyrptorchidism (undescended tested) and hypospadias (urethral opening is on the underside of the penis) are two anatomical abnormalities of the male genitalia which may impair male fertility. In undescended testes, the increased testicular temperature is responsible for the reduced fertility while in hypospadias; the retrograde ejaculation of semen is responsible for the infertility. Engorgement of the testicular veins leads to a varicocoele which also contributes to male infertility
Environmental: Exposure to toxins, chemical and free radicals are also responsible for reducing male fertility. Some estrogen like chemical compounds used as pesticides have show n to cause imbalance of the male hormones and also induce damage to the sertoli cells which are essential during the early development of the sperms.
Infections: Repeated infections with organisms like Chlamydia trachomatis or Neisseria gonorrhea causing sexually transmitted diseases are likely to produce infertility in males. Human papilloma virus can also compare with the affect the function of sperms contributing to infertility. Mumps is another viral disease which can cause infertility in 25% of men who have suffered from this infection. Also, chronic infections of the prostate, epididymis, seminal vesicles and urethra are likely to contribute to male infertility.
Noninfectious disorders: Diabetes is an important condition known to cause infertility in males mainly due to damage to the spermatic DNA. Chronic anemia, chronic renal failure and chronic liver disease also cause hormonal imbalance leading to male infertility.
Drugs: A large number of drugs prescribed for the treatment of various medical disorders are also responsible for impairing male fertility. Drugs like cimetidine, alcohols, colchicines, methotrexate, corticosteroids and calcium channel blockers have been implicated in causing infertility.

Radiation induced toxicity is also well known. Low doses of radiation (< 600 rads) causes reversible damage to the germ cells in the testes which takes two to three years to recover. Large doses of radiation (>600 rads) causes permanent germ cell damage. Such a situation is often seen in individuals receiving radiotherapy for cancers. Chemotherapeutic agents also are likely to affect male fertility.