Different causes for infertility in men:
Chemotherapy can reduce the number of sperm cells and their mobility. This can lead to temporary or permanent infertility. Discuss possible consequences with your physician. If you want children, you can have sperm frozen. Chemotherapy can cause difficulties in maintaining an erection. Chemotherapy can damage the chromosomes. Men who receive chemotherapy should therefore use contraceptives to prevent pregnancy. In children of men who conceived a child after their treatment, no more abnormalities were detected than in children of men who did not undergo chemotherapy.
Quality of sperm
The motility of the sperm cells is important. The sperm cells must be flexible enough to move through the mucus from the cervix, through the uterus and the fallopian tubes to the ovum in the end of the fallopian tube. In case of insufficient motility of the sperm cells, we speak of asthenozoospermia (poorly moving sperm cells). In most men with non-optimal sperm there is a combination of a low number, a low mobility and many abnormal forms of the sperm cells. A pregnancy can also occur with a reduced sperm quality, although the chance is smaller and it takes longer. Normally, 100 to 200 million sperm cells are released during ejaculation. Per milliliter there are about 20 to 50 million. Less than 20 million sperm per milliliter is called oligozoospermia (low sperm count). Sometimes there are no sperm cells in the sperm at all. This is called azoospermia (absence of sperm cells).
The testes have a temperature of 35 degrees. That is lower than the body temperature (37 degrees). This temperature is important for optimal seed production. Wearing tight underwear or often taking a very hot bath can increase the temperature, also when using a sauna. With a varicose vein in the scrotum the temperature of the test balls is sometimes increased.
Smoking and Drugs
Men who smoke make fewer sperm cells, less sperm fluid - the 'ejaculation' is smaller - and the sperm cells often have a different shape. Smoking can have an unfavorable effect on the quality of sperm. The same applies to the use of drugs.
Excessive use of alcohol (more than two glasses a day) may adversely affect the quality of the sperms.
Frequency of ejaculation
Too few or too many ejaculations in a given time can reduce sperm quality. In general, the 'saving' of sperm does not promote quality for a longer period of time.
After a disease with a fever, the quality of the sperm can suffer. The development and ripening of sperm cells takes about two to three months. So it may take some time before the quality is restored after a febrile illness.
A low production of follicle stimulating hormone (which stimulates the testicles) can lead to poor quality of the sperm, but this hormone disorder is very rare.
From angelababy In Q&A