Of course, meaningful probability rapidly becomes complex the more events or conditions we add.

We could assign a probability to condition A (developing AIDS) based on global data for condition A (acquiring) HIV.

But that wouldn’t tell us much, not if we lived in the US and had access to testing and treatment.

We would need to add other conditions if we wanted answers about our specific circumstances.

Condition C, D, E, etc.

Because the probability of an HIV-positive person living in rural Botswana developing AIDS is very, very high. It used to approach 1, though that’s backing off some finally.

The probability of an HIV-positive person living in NYC developing AIDS approaches zero very closely.

Rural Alabama? Way better than Botswana. Way worse than NYC.

Also … working out the probabilities depends on access to data, which may or may not be possible.

Lots to think about!