Granit Xhaka contracted COVID-19 after refusing to be vaccinated against the virus.
The Swiss Football Association (SFV/ASF) confirmed the news after the Arsenal midfielder missed Wednesday night's 2-1 friendly win over Greece with the virus.
They informed the media, "Xhaka was diagnosed with symptoms on the morning of the game, whereupon he was immediately isolated in his room.
"A quick test turned out negative, the subsequent PCR test, carried out to be on the safe side, produced a positive result in the evening."
The 28-year old is now likely to miss Switzerland's upcoming World Cup qualifiers with Italy and Northern Ireland as a result of his diagnosis.
During half-time of the Greece friendly, SFV communications officer Adrian Arnold was asked by Blick if Xhaka had been vaccinated against COVID-19.
“Granit Xhaka was not vaccinated," was his response, "He's a player who isn't vaccinated.
"We left this up to each player, it's a personal decision of each player - just like any other person in Switzerland.
"We have issued a recommendation that everyone vaccinate. But he decided for himself personally. And it is also his right not to be vaccinated."
The sequence of events continues a calamitous start to the season for Xhaka.
He captained Arsenal for their first two Premier League games of the season - defeats to Brentford and Chelsea - and was sent off in the 5-0 hammering at Manchester City last weekend.
Arnold added, "All the other players in the team have been vaccinated or have recovered - so they are more or less safe, at least from a medical point of view.