The illness is not airborne but is transferred by coming in contatct with his body fluid- from what I heard......
It gets airborne when the persons coughs and small bits get into the air. If my memory serves me right, that is how it is the main way it is spread. That is why they are looking for the people who sat close to him (and not everyone) on those planes.

He could have wore a surgical mask and not took a chance on spreading it.... but if he had done that, they would have asked why...