Infectious diseases 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. Infectious diseases that spread from person to person are said to be contagious.
Some infections spread to people from an animal or insect, but are not contagious from 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 strep throat) 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. And sexually transmitted diseases (STDs) are spread through all types of sex — oral, anal, or vaginal.
You can help protect yourself against contagious diseases by washing your hands well and often, staying away from those who are sick, making sure you're up to date on all vaccinations, and always using condoms during any type of sex.