Inability to conceive (in a woman) or to induce conception (in a man). About 10 to 15 per cent of all married couples are infertile, temporarily or permanently. In 40 per cent of cases the man is responsible, in 40 per cent the woman; in the remaining 20 per cent neither is clearly responsible. In most cases (8 out of 10) there is a physical cause; in the man these include imperfectly functioning testicles, caused by mumps virus infection, for example, venereal disease such as gonorrhoea, or congenital testicular deficiency. Sometimes a prostate operation, severe diabetes mellitus or nerve damage can cause semen to enter the bladder on ejaculation, instead of being forced out through the penis. Sperm deficiency is another common cause of male infertility. In women infection of the Fallopian tubes, ovaries or uterus can impede conception by the formation of scar tissue. Cervical inflammation prevents the passage of semen; the cervix may also secrete substances that kill sperm. Hormonal changes can prevent release of an ovum or the implantation of a fertilized ovum.