Don't want to share issues with the world

I'm having trouble dealing with people who want me to discuss an emotionally difficult topic. About a year ago, my father was diagnosed with a terminal illness. He became ill suddenly and spent several days in the hospital. I told a few people about his condition (mostly because I had to miss work and cancel some plans), but word spread, and people I'm not close to found out.

Since that time, I'm often asked about my father's condition, and it throws me for a loop. People often bring it up during large social gatherings when I don't feel like talking about something emotionally heavy (I never bring it up myself). I try to deflect by saying, "He's doing fine, thanks for asking," and changing the subject, but more often than not people -- often those whom I never told directly -- will keep pressing me for more information. I get emotional when I have to explain what's going on, and I feel pretty down afterwards. Sometimes I'll even be chastised for not being more open, such as being told that it's not healthy for me to keep my feelings inside. I do have some close friends I talk to about my father, but I don't want to talk to everyone all the time. What is a good way to explain this to someone and get them to leave me alone?

Being honest is the best way to. If you do want to talk about it just say so.