Javi Gracia has revealed that the Watford camp has been hit by a stomach virus, with four players currently ill.
The Hornets, who host Crystal Palace in the Premier League on Saturday, are nine points above the relegation zone, but have lost four of the last five games.
Forward Gerard Deulofeu and defender Sebastian Prodl have both been absent from training due to the illness sweeping the camp, alongside two additional unnamed players.
"Gerard was sick this week and there were other players who have been ill, too," Gracia told reporters. "Prodl has also been ill all week.
"It is a stomach virus. They are recovered, available for the match and I am sure they are in the best condition."
Watford, currently 12th in the standings, will be overtaken by Palace if the Eagles win at Vicarage Road on Saturday.
