Three Catalans Dragons players and one member of the non-playing staff have tested positive for COVD-19.
The Dragons squad were all instructed to isolate as soon as news of the positive tests were received and they will have further tests later in the week.
Saturday's Super League match against Wigan has been postponed.
Two of the Dragons players who have the virus played against Wakefield last weekend and, as a result of the test and trace analysis, two of Trinity's players also have to self-isolate.
A Super League statement read: "Catalans Dragons' Betfred Super League fixture against Wigan Warriors, which had been scheduled for this Saturday (August 29), has been postponed, after three Dragons players and a member of their non-playing staff returned positive tests in routine Covid-19 testing today (Tuesday).
"Wigan Warriors will now play Castleford Tigers, in a fixture which had been due in Round 18.
"Castleford had been due to have a bye this weekend, but all clubs with a bye have been instructed to be on stand-by to step up in such an eventuality, given the exceptional current circumstances.
"The Dragons squad were all instructed to isolate as soon as the news of the positive tests was received, prompting the postponement of the Wigan fixture.
"They will have further tests later this week. Of the players who have tested positive, two played in Saturday's Coral Challenge Cup tie against Wakefield Trinity.
"Test and trace analysis has shown that two of the Wakefield players will also be required to self-isolate - but at this stage their fixture against Warrington Wolves is expected to go ahead as scheduled on Sunday."