Flight time from Liberia to Nigeria is 2 1/2 hours. He had to have known he was infected before he boarded. Just saying.
Sawyer had been caring for his sister, who died from Ebola.
You are right; he knew he had been exposed and surely knew he was a risk to others. He should never have boarded that plane.
Before symptoms break out, are you generally aware that you have been exposed to a disease?
Ebola can mimic many other diseases in the early stages. And Africa has more than its share of fever inducing diseases.