This guy new the family had been exposed to Ebola. That is why he was there. I doubt he touched anything.
How did the Sheriff get in without touching the doorknob? (no one wore gloves or protective gear for the first day or so).
Did he lay the papers on a table? Ebola can live on a dry surface for 6 days.
Has the virus become aerosolized (thanks to the dear departed Mr. Duncan)?