He was born of a 'virgin' due to a poor translation of the Hebrew Bible into Greek. The Hebrew word in Isiaih (?) was 'young girl' which was translated into Greek with a word that meant either 'young girl' or 'virgin'. The early Christians used the Greek translation and settled on the 'virgin' meaning when they wanted to show that Jesus fulfilled the Hebrew prophesies.
Further, the Torah does have a specific word for a virgin – betulah – which it consistently uses in reference to a woman’s virginity (e.g., Genesis 24:16, Exodus 22:15, Leviticus 21:14, and especially Deuteronomy 22:13-21) – and that is not the word used by Isaiah. Matthew 1:23 claims that Jesus’s immaculate conception was the fulfillment of Isaiah’s prophecy that a virgin shall give birth, but it is immediately clear that this is not what Isaiah had in mind.