Originally posted by Admiral Akbar
He coughs, not because he is alien, but because Mace Windu smashed his plate, with the force.
This I always took for granted to be true. Recently I found a site (it seemed pretty credible - maybe it was Wikipedia) that confirmed my belief. It just makes sense, anyways, since Grievous coughed on the ship in ROTS but not when he fought Obi-Wan Kenobi. When he was on the ship, it was a takeoff directly from Clone Wars, when Palpatine was taken captive, so it made sense that the coughing was a result of Mace Windu's Force Crush on Coruscant, seeing as Grievous was hacking right afterward in Clone Wars.
As pointless as that may have seemed, I thought it worth mentioning. Peace be with everyone.