Probability is amazing.

And counter-intuitive at times.

Say there is a 1/1000 chance that an infant would have spina bofida. And let's say there's a test that never tests negative when the child has actually has it and 95% of the time tests positive when the child doesn't actually have the disease.

The question is, given that the child tests positive for spina bofida, what is the probability that the baby truly has the disease?

The surprising answer is approximately 2%.

So if the baby tests positive and you're one of it's parents, it shouldn't phase you all that much.