Despite his previous encounters with God, Abraham chose to trust his own instincts when he journeyed into Egypt, and misled Pharaoh by presenting Sarah as his sister. Pharaoh and his household got sick because he had taken Sarah, the wife of Abraham, as his concubine. Up until the affliction arose, Abraham benefited greatly from this sinful arrangement as he received generous favors from Pharaoh, in appreciation for Sarah's company.
It would seem that God chose to afflict Pharaoh