The Jews connected suffering with sin. Rabbi Ammi stated, “There is no death without sin, and there is no suffering without iniquity” which he proved from (Psalms 89:32). (qt’d by Morris)  The Book of Job had shown that this was not always the case and Jesus had also stated it (Luke 13:1-5), but the view was still widely held and the disciples were perplexed by it. There were two possibilities for who sinned to cause this blindness: the man himself or his parents.

Psalms 89:32  I will punish their sin with the rod, their iniquity with flogging;

Luke 13:1-5  Now there were some present at that time who told Jesus about the Galileans whose blood Pilate had mixed with their sacrifices.  Jesus answered, “Do you think that these Galileans were worse sinners than all the other Galileans because they suffered this way?  I tell you, no! But unless you repent, you too will all perish.  Or those eighteen who died when the tower in Siloam fell on them—do you think they were more guilty than all the others living in Jerusalem?  I tell you, no! But unless you repent, you too will all perish.”

• the man – Since he was blind “from birth” he must have sinned while in his mother’s womb. Based upon (Gen 25:22), the Jews accepted this as quite possible.
  Genesis 25:22  The babies jostled each other within her, and she said, “Why is this happening to me?” So she went to enquire of the LORD.
• his parents – This concept is quite Biblical and ran throughout the Old Testament (Ex 20:5, 34:7, Num 14:18, Jer 32:18). We must always remember that our sin doesn’t just affect ourselves: it hurts others. Barclay states it thus: “When a man sins he sets in motion a train of consequences which has no end.” Can you give some examples of the sins of parents that will cause their children to suffer?
  Exodus 20:5  You shall not bow down to them or worship them; for I, the LORD your God, am a jealous God, punishing the children for the sin of the fathers to the third and fourth generation of those who hate me,
  
  Exodus 34:7  maintaining love to thousands, and forgiving wickedness, rebellion and sin. Yet he does not leave the guilty unpunished; he punishes the children and their children for the sin of the fathers to the third and fourth generation.”
  
  Numbers 14:18  ‘The LORD is slow to anger, abounding in love and forgiving sin and rebellion. Yet he does not leave the guilty unpunished; he punishes the children for the sin of the fathers to the third and fourth generation.’
  
  Jeremiah 32:18  You show love to thousands but bring the punishment for the fathers’ sins into the laps of their children after them. O great and powerful God, whose name is the LORD Almighty,