He could have caught it from Duncan or when he traveled to West Africa himself. What, has half of America been to West Africa in the past few weeks???
http://www.dailymail.co.uk/news/article-2785514/Second-person-rushed-hospital-Ebola-symptoms-contact-patient-zero-Thomas-Duncan.html
Why did he check yes to traveling to West Africa? In error? Or was he really there?