Dear Doctor: It isn’t always easy to tell flu from a cold. Here are some things to look for

DEAR DR. ROACH: What is the difference between a cold and the flu? -- A.T.R.

ANSWER: Both colds and influenza (the “flu”) are respiratory illnesses that are caused by viruses. Influenza is caused specifically by the influenza viruses, which come in two main types (A and B). Influenza A is further broken down into different subtypes based on hemagglutinin (H) and neuraminidase (N).

H1N1 and H3N2 are currently the most-common circulating influenza A viruses. Even within these subclasses, there is strong variability among flu viruses, which is one reason why we need a new flu vaccine each year.

Many viruses cause colds. Rhinoviruses (“rhino-” is the Greek word for nose) are the most common, but corona-, parainfluenza-, entero- and respiratory syncytial viruses are all well-known causes of