Leicester's efforts in the 2-2 FA Cup draw at Derby were affected by a number of players suffering from a stomach bug.
Wes Morgan, Robert Huth, Danny Simpson and Jamie Vardy were among those to battle against significant ill effects.
Leicester believe the issue will have cleared up by Tuesday's trip to Burnley but the virus appeared to undermine performance levels at Pride Park, when Morgan's header four minutes from time averted an upset.
Claudio Ranieri said: 'It was very important not to lose, we fought a lot to equalise.