Bundesliga side Hoffenheim on Wednesday put their squad in quarantine after four players tested positive for the coronavirus in the previous 24 hours, while their goalkeeper Oliver Baumann has left the Germany camp as a precaution, the German FA announced.
"We cannot explain the cluster," said Hoffenheim sports director Alexander Rosen.
"We have been meticulously and conscientiously adhering to the hygiene concept for months.
"We came through the pandemic without a single positive case for months, which is now hitting us very hard."
German midfielder Sebastian Rudy and Algerian forward Ishak Belfodil, plus a member of their coaching staff, have tested positive for Covid-19, Hoffenheim said Wednesday.
It comes after Israeli striker Munas Dabbur and Danish winger Robert Skov tested positive for the virus on Tuesday with their national teams.
In total, Hoffenheim has seven cases of the coronavirus after Danish forward Jacob Bruun Larsen and a club member of staff also tested positive in recent days.
Hoffenheim goalkeeper Baumann, due to be a replacement for Germany in Wednesday's friendly against the Czech Republic, has twice tested negative with the national team, but the German FA (DFB) sent him home to be quarantined as a "precautionary measure".
"Baumann will not be available for the coming internationals," the DFB said in a statement with Germany due to play Ukraine in the Nations League on Saturday, then Spain away next Tuesday.
His club Hoffenheim is now in close contact with the local health authorities to contain the outbreak.
Last week, Hoffenheim played in the Europa League against Czech club Slovan Liberec, who were missing 15 players sidelined by the coronavirus.
Due to the current international break, Hoffenheim's next Bundesliga game is scheduled for November 21 at home to Stuttgart.
During October's international break, both Croatia striker Andrej Kramaric and Ghana defender Kasim Adams returned from their national teams with the coronavirus.
Hoffenheim's Czech right-back Pavel Kaderabek was quarantined after a positive case in his family, but the trio had all since returned to training.
Hoffenheim are the second Bundesliga club to go into quarantine after Werder Bremen's squad were briefly kept at home last month following a single positive test.
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