Dear Dr. Roach: My brother-in-law has had blood clots in his legs twice before. Now he has another one in his calf, and tests showed several in his lungs! What causes blood clots, and how can you prevent them? M.K.E.
Answer: The body must have a robust ability to stop bleeding, which needs to be ready at any time. That ability has a cost: possible inappropriate clotting inside the body.