If you haven't had chickenpox as a child and catch it later when on immunosuppression drugs this can be a life-threatening situation. Chickenpox can kill healthy adults who don't have immunity to it. It can certainly kill immunosuppressed ones who haven't had it before.
I started Azathioprine and steroids in Spring 1998. A few weeks after I woke up one morning and was covered in spots. I feared they were chickenpox, though I'd never had it before, called the doctor's surgery right away, and a GP came round within 15 minutes. She said yes, it was chickenpox, and phoned up the infectious diseases unit at the nearby city, and arranged for me to be admitted there immediately.
I was told I only just got there in time. My pulse was already stratospheric (every time a computer checked it an alarm would go off), but luckily I didn't develop anything worse. I was on anti-viral drugs, intravenously, for a week. That was actually very painful because I showed signs of vasculitis in my hands where the drips were going in - the only time I have ever showed signs of vasculitis outside my brain. But the drugs worked. After a week I was discharged, somewhat reluctantly, because my pulse was still quite impressively high. But I was desperate to get home, to my own bed, where I could sleep properly. Sleeping as an in-patient in hospital can be difficult.
More recently I developed shingles, where the chickenpox virus reactivated. That was very painful, horrible nerve pain, running down a nerve from my waist down my left leg to my toes. Quite agonising. And a much nastier rash than I had with the chickenpox. People who are immunosuppressed are at a greater risk of the virus reactivating like this. They can also get shingles repeatedly. Be careful that shingles is infectious. You can't give someone shingles, but if someone hasn't had chickenpox before they can catch chickenpox from someone else who has shingles. Shingles is infectious until the rash starts to crust over and dry up. It should be covered up at all times.
So if you are immunosuppressed and haven't had chickenpox before and someone in your family develops either chickenpox or shingles please keep away from them. And contact your GP for advice. You may be advised just to look out for signs of the disease developing. But if you do develop spots seek urgent medical help.