In: Statistics and Probability
Virus Z is a new virus infecting people. Some people get Virus Z and develop no systems. In others, Virus X causes a disease called Disease Z.
If you have Virus Z, you have it for life and everyone you touch gets Virus Z, if they didn’t have it already. (They get it immediately when you touch them.) If you get Virus Z, you will either immediately develop Disease Z, or you will never develop Disease Z, and those two outcomes are equally likely. Different people’s reactions to the virus are independent.
There is a 50% chance Bill has Virus Z. Bill does not have Disease Z. Bill touches four people without Virus Z. Given that the first three do not develop Disease Z, what is the probability that the fourth one develops Disease Z?