Ulster's clash with the Sharks has been postponed after Dan McFarland's squad was hit by a gastroenteritis bug.
The province were due to take on the Sharks in Durban on Saturday evening in their latest United Rugby Championship fixture.
The Glasgow Warriors' game against Emirates Lions is also off after a similar bug was reported in the travelling Scottish camp.
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Ulster said in a statement: "The URC has this afternoon confirmed that the Round 6 fixtures between Cell C Sharks v Ulster, and Emirates Lions v Glasgow Warriors, are postponed.
"The Round 6 United Rugby Championship fixtures involving Ulster and Cell C Sharks, and Glasgow Warriors and Emirates Lions, have been postponed.
"Due to gastroenteritis infections, the United Rugby Championship have confirmed that it is not possible for Ulster or Glasgow Warriors to field full playing squads.
"In order to ensure a duty of care towards the players involved, and given the medical advice received from all key stakeholders, the United Rugby Championship has made the decision to postpone the fixtures."
Ulster were targeting a big double after beating the Lions last Saturday in what was their first URC win in South Africa.
The league will now examine potential dates to reschedule both games.
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