What do you think about the fact that I'm not going to tell someone they have a potentially fatal illness?
Are you serious? Why on Earth would you not tell them that they have a potentially fatal illness?
Because I want to tease them, also I don't have a lot of concern for them
If you know that someone has a serious illness then you have no right to keep that information from them. It's extremely wrong to hide information about someone's health from them.
I'm going to tell them they have cancer but I'm not going to tell them they have AIDS. I'm only doing it as a joke.
Joking about someone having extremely serious diseases is awful, and you should absolutely not do it. If you're certain they have a particular illness then you should let them know, otherwise don't say anything.