It had been suggested that the Chelsea defender left the Three Lions camp with minor illness, but the 23-year-old took to Twitter to insist his condition is far more severe.
"Just to clarify.... Its not a "sore throat" Im ill. Swollen glands in my neck... And constant headache for 3 days now and freezin cold," Bertrand tweeted.
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"Do you think a "sorethroat" could stop me being apart of a match for my club or country #yourf***ingnuts this is what every boy dreams of."
Bertrand has enjoyed a breakthrough year at senior level, appearing in last season's Champions League final.
He went on to represent Team GB at the Olympic Games before earning his first England cap in a friendly against Italy back in August.
His tweets regarding the matter have since been deleted.