News of this illness was reported by the German federation, per The Guardian.
"Schurrle suffered a persistent infection in October and could not train regularly," read the statement. "It was agreed with Joachim Low that he stays in London and trains individually to optimise his fitness."
Such a realisation may shed light on why the creative winger has struggled to hold down a regular spot in Jose Mourinho's Chelsea team. Schurrle failed to make the bench against Liverpool and hasn't started a Premier League match since the 2-0 win over Arsenal on October 5.
He managed a below-par 90 minutes during the Capital One Cup tie with Shrewsbury on October 28 but has been forced into bit-part performances across the entire season. Mourinho has started Schurrle in just four domestic league matches, with his most recent start coming during a 45-minute appearance against Maribor in Europe.
The player is currently linked with a swap deal that would see Borussia Dortmund's Marco Reus head to Stamford Bridge, a move widely reported in the press.
While it is premature to suggest Schurrle's presence is no longer needed, the 24-year-old faces a battle against fitness and form to retain a starting spot in the West London side.