circumstances. your kid or you are exposed to somebody with an infectious disease and you don't get it. you say ah-ha, my immune system protected me. so there's no way to test your immune system. >> jamie: ok, doc, thank you. >> eric: a moment ago, the doctor discussed the importance of stress tests when it comes to heart disease. coming up, dr. rosenfeld will tell us about possible link between testosterone and heart disease. plus defiant attorney general eric holder. he's on the defensive saying he never misled congress about the fast & furious gun operation. coming up in the next hour, we'll talk to an idaho congressman about why he's calling for holder to quit! do you have an irregular heartbeat called atrial fibrillation, or afib, that's not caused by a heart valve problem? are you taking warfarin to reduce your risk of stroke caused by a clot?