What's the difference between infectious and contagious?

Infections are diseases that are caused by microscopic germs (such as bacteria or viruses) that get into the body and cause problems. Some — but not all — infectious diseases spread directly from one person to another. Infections and other diseases that spread from person to person are said to be contagious.

Some infections spread to people from an animal or insect, not another human. Lyme disease is an example: You can't catch it from someone you're hanging out with or pass in the street. It comes from the bite of an infected tick.

Contagious diseases (such as the flu, colds, or other infections) spread from person to person in several ways. One way is through direct physical contact, like touching or kissing a person who has the infection. Another way is when an infectious microbe travels through the air after someone nearby sneezes or coughs. Sometimes people get contagious diseases by touching or using something an infected person has touched or used — like sharing a straw with someone who has mono or stepping into the shower after someone who has athlete's foot.

Even if an infection or other disease is contagious, being exposed to it does not mean a person will automatically get sick. Our immune systems do a great job of warding off the germs that cause many diseases, often because we've had the disease before or because we're up to date on our shots!