From the article "Know your chances" in Scientific American:

"Consider a woman who has just received a positive result from a mammogram and asks her doctor: Do I have breast cancer for sure, or what are the chances that I have the disease?

• The probability that a woman has breast cancer (prevalence) is 1 percent.
• If a woman has breast cancer, the probability that she tests positive (sensitivity) is 90 percent.
• If a woman does not have breast cancer, the probability that she nonetheless tests positive (false-positive rate) is 9 percent.

What is the best answer to the patient’s query?

1. The probability that she has breast cancer is about 81 percent.
2. Out of 10 women with a positive mammogram, about nine have breast cancer.
3. Out of 10 women with a positive mammogram, about one has breast cancer.
4. The probability that she has breast cancer is about 1 percent."

If your answer was not number 3, then read this article. Seriously, I mean it.