The suit, which seeks class-action status, names six Alden facilities: three in Chicago and one each in Cicero, Harvey and McHenry.
Combined, they house about 1,400 people.
Attorneys held a news conference Wednesday and called the understaffing "systematic" and "intentional."
The nursing home company responded with this statement to ABC7, "Alden vigorously denies any and all allegations of wrongdoing set forth by the plaintiffs and their attorneys and is confident the judicial process will vindicate Alden in this action."