Watford's recent troubles deepened this week after four players were struck down with a stomach virus.
The Hornets, who host Crystal Palace in the Premier League on Saturday, are looking over their shoulder after losing four of their last five matches.
Gerard Deulofeu, the forward on loan from Barcelona, and defender Sebastian Prodl have both been absent from training due to the illness sweeping the camp.
Manager Javi Gracia did not wish to reveal the names of the other two players who were affected, but hopes all four will be fit for the visit of Roy Hodgson's side.