Gareth Taylor and Matthieu Louis-Jean missed Tuesday's 4-1 drubbing by Walsall and remain doubts for the game with the Hornets.
Darren Ward, Marlon Harewood, Eion Jess and Danny Sonner have now also gone down with the bug which could throw Paul Hart's selection process into the air.
Forest's squad is already down to the bare bones with David Johnson out until the New Year and Michael Dawson suffering from a hamstring injury.