The condition was diagnosed following tests after the Bulgaria international, 32, developed a fever following last Saturday's defeat to Arsenal.
A statement from the club said: "We expect to learn more about Stiliyan's situation in due course and we have moved quickly to support him and his family. During this time we ask that Stiliyan's privacy is placed ahead of all inquiries and trust that we will share information as we receive it."
It continued: "Stiliyan is cherished by many and he will get from Villa every ounce of love and support that we have to help bring this to a positive conclusion."
Source: PA
Source: PA