Dear Doctor: What’s the difference between a cold and the flu, and how do you know which one you’ve got? Why can’t antibiotics help? Dear Reader: Whether it’s a cold or the flu, the reason you are feeling awful is because you’ve been infected by a virus. More than 200 different types of viruses can cause a cold, and the most common is the rhinovirus, the culprit behind up to half of all colds. Th…