I'm pregnant and by boyfriend has cancer, how did this happen?

We were told that he was sterile and could not have children because he was undergoing chemotherapy. Yet, I just found out that I am pregnant. There is no question that it is from him because I have been with no body but him but how could this have happened? We're both 19 and I am away at college six hours away. And the timing is right to be his regardless because he visited me 2 and a half weeks ago and I am about 2 and a half weeks pregnant. How did this happen and how would the cancer affect the baby?