I feel so guilty, please help.?

Basically my friend told me quite a while ago that she had cancer, although her mum won't tell her where about because she doesn't want her to look it up and get paranoid about it, but anyway... She's had it for quite a while now and I still feel terrible about it. I really wish I could do something about and I asked a doctor if there's any way I could help with cancer and he said no, all she needs is support. but back to the real story. She has a lot of other problems, like she's enemic, dyslexic and has a shortage of blood cells. So last week we were going home on the school bus and I wanted to get her back as a joke because she coughed on me a lot on purpose (she had a cold and gave it to me), as a joke. So I did it to her on the bus (as a joke and I had a cold) and I didn't know how that would effect anything. Turns out, the week we were off, she gave it to her brother and her brother gave it back but worse. So basically the cold I gave her as a joke made her sick. The kind of sick when ur just when you're generally ill, but she told me it effected her cancer and she had to take injections and stuff. Apparently I've made it worse and the guilt is literally eating me alive. Now I just wish I could die. I want to punish myself. Actually, I have. I feel like crying I've made everything worse. It's all my fault. Shes forgiven me but I will never ever forgive myself. Ever. What I did was horrible and selfish. How can I live with all the guilt. And is it true that giving someone a cold but making them sick affect cancer? What will happen? Please help! My stomach has filled to the brim with guilt and hatred for myself. I'm a monster. Why can't I suffer instead of her.