Gym contract

On the 28th of February 2019 I joined Virgin Active gym and was signed up for a 2 year contract. After every session at gym I will feel dizzy and tired so I decided to visit the doctor after various tests, my doctor found that I have Iron deficiency due to my body not absorbing iron. The doctor advised me to stop training and he gave me a letter to submit at the gym. I was hoping that virgin active can cancel the contract without any penalties. I then received a call from virgin active advising me that they can only cancel my membership with a minimum fee of R1780. My concern is that I'm forced to stop training due to health issues why can't they cancel my membership without any penalties? Please assist me in this matter