The first night is their flat in Ireland, why did eugene wake up screaming?

Eugene woke up screaming the first night in their flat because he was being bitten by fleas.

Then Eugene sat up, screaming, tearing at himself. Ah, ah, Mommy, Mommy. Dad sat up. What? What's up, son? Eugene went on crying and when Dad leaped from the bed and turned on the gaslight we saw the fleas, leaping, jumping, fastened to our flesh. We slapped at them and slapped but they hopped from body to body, hopping, biting. We tore at the bites till they bled. We jumped from the bed, the twins crying, Mam moaning, Oh, Jesus, will we have no rest! Dad poured water and salt into a jam jar and dabbed at our bites. The salt burned but he said we'd feel better soon.