Too many variables to isolate on this study to provide proof that correlation equals causation. Even with the isolating they've tried to do with the 95% CI the range is:

1 Person Office: 1 (They didn't report a range here)

2 Person Office: 1.13-1.98 sick days

3-6 Person Office: 1.08-1.73 sick days (LOWER than 2)

6+ Person Office: 1.30-2.02 sick days

Great item to study further. This isn't scientific proof of anything. As a solo worker you may come in to the office when sick as well since you don't have to worry about getting anyone else sick and so many other variables they didn't control for make this interesting but far from proof.

Direct PDF link to full study: http://www.sjweh.fi/download.php?abstract_id=3167&file_nro=1