The 23-year-old began to suffer from a bacterial infection after arriving at Villa Park from Aalborg in the summer of 2013, with the condition becoming so serious that the player feared for his life.
"It ruined me physically because the bacteria couldn't get out and I couldn't get the vitamins I needed," Helenius is quoted as saying by The Guardian. "I thought I had a very serious disease or even cancer.
"I had very bad thoughts and was afraid of dying and those thoughts of dying were with me all the time. I was crying for help. I had to get through this and that's why I went back to Aalborg to feel comfortable in Denmark.
"I was addicted to [sleeping pills]. I was almost like a drug addict, but these pills are not good for your body."
Helenius saw only 147 minutes of competitive action for Paul Lambert's team before returning to Aalborg in August on a season-long loan.