first thing you should know is- infertility means unable to concieve even after one year of unprotected regular Sex... A woman gets pregnant around the days when ovulation occurs. The average time of ovulation is the 14th day of an average length (28 day) menstrual cycle. Ovulation time vary for each individual, and can occur from 12th to 19th day of the cycle . Sperm can live up to 3 to 5 days in a womans reproductive tract, so it is possible to become pregnant if unprotected sex occurs 5 days before ovulation. The life span of the typical egg is relatively short, only around 24 hours. Low sperm count may be due to psychological reason as well as physical reason...physical causes are many, it may be due to trauma to the testis, infections,obesity,kidney diseases,liver diseases,certain medications mainly pain medications derived from opioids,people suffering from sickle cell disease or thalassemia can also have this,people taking glucocorticoids can also have this,other hormones in the body with abnormal levels can also impact on testosterone levels,chronic alcoholism is also one of the cause....I would suggest you to consult nearest infertility specialist they will do the needful.