Complex sentence: For example, if you have cancer you might have a 90% chance of testing positive for cancer, but if you test positive for cancer you might have only a 10% chance of actually having cancer because cancer is very rare.
Split into simpler sentences: For example, if you have cancer you might have a 90% chance of testing positive for cancer. Alternatively, you can test positive for cancer but you may have only a 10% chance of actually having cancer because cancer is very rare.